3DG J34 Pressure Measurement Test

BECHTEL CORPORATION ENGINEERING ENGINEERING - CONTROL CONTROL SYSTEMS SYSTEMS ENGINEERING DESIGN GUIDE PRESSURE MEASUREMENT 3DG-J34-00001, Revision 002 2002 August 05 Reason for Revision: Re-Issued for Use Prepared by: B. Parkes Checked by: M. Benefer Approved by: A. P. DiMartino

INTRODUCTION This Design Guide covers the requirements for pressure measurement and the application of pressure measuring instruments.

Electronic documents, once printed, are uncontrolled and may become outdated. Refer to the electronic documents in BecRef for current revisions.

Bechtel Confidential © Bechtel Corporation 2002. Contains confidential and/or proprietary information information to Bechtel and its affiliated companies which shall not be used, disclosed or reproduced in any format by any non-Bechtel party without Bechtel’s prior written permission. All rights reserved.

3DG-J34-00001

Page 1 of 28

CONTENTS

INTRODUCTION

1

1.0

PURPOSE

3

2.0

CODES AND STANDARDS

3

3.0

PRESSURE MEASUREMENT

4

4.0

PRESSURE GAUGES/INDICATORS

6

5.0

PRESSURE TRANSMITTERS

19

6.0

PRESSURE SWITCHES

22

7.0

PRESSURE INSTRUMENT INSTALLATION

24

8.0

WORK PROCESS

25

9.0

REFERENCES

28

Electronic documents, once printed, are uncontrolled and may become outdated Refer to the electronic documents in BecRef for current revisions. Bechtel Confidential © Copyright Bechtel 2002. All rights reserved.

3DG-J34-00001

Page 2 of 28

1.0

PURPOSE

The purpose of this Design Guide is to guide the instrument engineer in the selection, specification and installation of pressure measuring devices. Pressure measuring devices are divided into three (3) categories: pressure indicator/pressure gauges, pressure transmitters and pressure switches. In the industrial world, pressure measuring devices are not only used to sense pressure but also to measure inferentially level and flow rate. These inferential measurements are usually made with differential pressure measuring devices. This design guide, 3DG-J34-0001, pertains to pressure and differential pressure sensing devices. The applications of pressure and differential pressure instruments in the measurements of flow rate and level are described in detail in design guides 3DG-32-0001 (Flow Measurement) and 3DG-J33-00001 (Level Measurement) respectively. 2.0

CODES AND STANDARDS

ASME B40.100a

Addenda to ASME B40.100 1998 for Pressure Gauges and Gauge Attachments which incorporates ASME B40.1 2000, ASME B40.2 2000, ASME B40.5 2000, ASME B40.6 2001and ASME B40.7.

ASME B1.20.1

Pipe Threads : General Purpose

ASME PTC 19.2

Part 2 : Pressure Measurement Instruments and Apparatus

API 551

Process Measurement Instrumentation (Section 7 Pressure)

ISO 228-1

Parallel Pipe Threads (G)

EN 472

Pressure Gauge Vocabulary

BS 6134

Pressure and Vacuum Switches

BS EN 837-1

Part 1 : Bourdon Tube Pressure Gauges - Dimensions, Metrology, Requirements and Testing

BS EN 837-2

Part 2 : Selection and Installation Recommendations for Pressure Gauges

BS EN 837-3

Part 3 : Diaphragm and Capsule Pressure gauges - Dimensions, Metrology, Requirements and Testing

Note: British Standard BS EN series of standards are equivalent to CEN EN series of standards and carry the same number e.g. : BS EN 837-1 is the same as CEN EN 837-1.


3DG-J34-00001

Page 3 of 28

3.0

PRESSURE MEASUREMENT

3.1

CATEGORIES

Pressure measuring devices are logically divided into the following categories: ¨

Pressure Gauges/Indicators

¨

Differential Pressure Gauges/Indicators

¨

Pressure and Differential Pressure Transmitters

¨

Pressure Switches

3.2

CLASSIFICATIONS OF PRESSURE AND ENGINEERING UNITS

3.2.1 Classifications of Pressure The four classifications of pressure are: ¨

Gauge Pressure

¨

Absolute Pressure

¨

Vacuum (Negative Pressure)

¨

Differential Pressure

This demarcation of various types of pressure developed because to sense each type of pressure requires a different type of pressure measuring device. In order to convert measurements between gauge and absolute pressure the value of the atmospheric pressure at sea level is added or subtracted to the given measurement. absolute pressure = gauge pressure + atmospheric pressure e.g. psia = psig + 14.696 (1 Atmosphere based on 60°F at sea level) gauge pressure = absolute pressure - atmospheric pressure e.g. psig =psia - 14.696 (1 Atmosphere based on 60°F at sea level) Note: Atmospheric conditions should be corrected for the site or plant location. Electronic documents, once printed, are uncontrolled and may become outdated Refer to the electronic documents in BecRef for current revisions. Bechtel Confidential © Copyright Bechtel 2002. All rights reserved.

3DG-J34-00001

Page 4 of 28

Differential pressure is the difference in pressure between two locations. It is calculated by subtracting the higher pressure from the lower pressure. Differential pressure units are psi and both high/low pressure points must be measured in the same units either absolute or gauge pressure. 3.2.2 Engineering Units The engineering units that are utilized to describe pressure are numerous. Listed below are the most popular English (Imperial) and metric units in use. English (imperial) units for pressure: ¨

Pounds per square inch (psi)

¨

Inches of water (in H 2O)

¨

Inches of mercury (in Hg)

In H 2O and in Hg must be corrected for temperature (generally 60°F since most liquids tend to expand on increasing temperature) Metric units for pressure: ¨

Bar (bar)

¨

Millibar (mbar)

¨

Kilograms per square centimeter (kg/cm )

¨

Pascal (Pa) or Kilo Pascal (kPa)

¨

Millimeters of mercury (mm Hg)

¨

Mllimetres of water (mm H 2O)

2

Conversion factors for these units related to the Pascal are as listed in Table 1.


3DG-J34-00001

Page 5 of 28

Table 1 Common Pressure Conversion Factors

4.0

Unit

Symbol

No of Pascals

Pounds per square inch

psi

6894.76 Pa

inches of mercury

in Hg

3386.39 Pa

millimeters of mercury

mm Hg

133.322 Pa

inches of water

in H2O

249.0889 Pa

bar

bar

1 x 10 Pa

millibar

mbar

100 Pa

kilograms per square centimeter

kg/cm

5

2

5

0.98067 x 10 Pa

PRESSURE GAUGES/INDICATORS

In order to select, specify, and install the most suitable pressure gauge for any given application the Instrument engineer must be knowledgeable with all the various aspects of pressure gauges: ¨

Function

¨

Case Type

¨

Accuracy

¨

Specific Type of Application

¨

Measuring Element

There are also liquid-filled gauges and many accessories are available that can enhance the performance of a pressure gauge. 4.1

FUNCTIONS OF PRESSURE GAUGES

Pressure gauges are specifically designed and fabricated and have specific indicating scales to meet the following pressure measurement requirements: ¨

Measure vacuum Electronic documents, once printed, are uncontrolled and may become outdated Refer to the electronic documents in BecRef for current revisions. Bechtel Confidential © Copyright Bechtel 2002. All rights reserved.

3DG-J34-00001

Page 6 of 28

¨

Measure gauge pressure

¨

Measure differential pressure

4.1.1 Pressure Gauges The common practice in instrumentation is that when the phrase “Pressure Gauge” is used it always implies that the instrument will measure gauge pressure, where 0 (zero) on the indicating scale denotes ambient or atmospheric pressure (0 psig) 4.1.2 Vacuum Gauges A vacuum gauge indicates “negative” pressure, usually having a span of 0 to 30 in Hg which is approximately the maximum vacuum (0 psia) that can be attained. 4.1.3 Compound Gauges A compound gauge is capable of indicating pressure above and below ambient pressure, that is, positive and negative pressure. The positive pressure portion of the scale is usually calibrated in Pa, bar or psi and the negative pressure or vacuum portion in psi or in Hg. 4.1.4 Dual Scale Gauges A dual scale gauge has a single pressure element, the dial of which contains a basic pressure scale and one or more additional concentric scales graduated in equivalent values of a different pressure unit or other parameters related to the basic pressure. 4.1.5 Retard Gauges In a retard gauge, the measuring element moves freely through only a portion of its pressure span, usually the lower portion. At a definite point in the upper portion of its pressure span, the rate of the remaining motion per unit of pressure change is reduced. 4.1.6 Surpressed Scale Gauges A gauge having a suppressed scale shows only the upper portion of the total pressure range. At pressures below the minimum shown on the scale, the indicating pointer is not actuated by the elastic chamber and remains inactive at the minimum point of scale until the pressure rises above the minimum value. A special gauge called a “receiver gauge” is an example of one having a suppressed scale. A receiver gauge is designed to indicate the output from a pneumatic transmitter and calibrated with reference to transmitter output. This type of gauge has a suppressed scale of 3-15 psig. The dial can be graduated in units related to the transmitter and can be in temperature, pressure, flow or level. Frequently this type of gauge has a dual scale, the first is the transmitted signal and the second the quantity of the measured parameter. Electronic documents, once printed, are uncontrolled and may become outdated Refer to the electronic documents in BecRef for current revisions. Bechtel Confidential © Copyright Bechtel 2002. All rights reserved.

3DG-J34-00001

Page 7 of 28

4.2

ENCLOSURE/CASE CATEGORIES

Pressure gauges are supplied in a wide variety of case types and are described by such characteristics as size, method of mounting, location of pressure connection, and general construction of case and ring. 4.2.1 Size Case sizes are available in standard sizes varying from 40 to 405mm (1-1/2" to 16") diameter. These diameters refer to the approximate inside diameter of the case at the dial level. The standard sizes are : 40mm (1 1/2"), 50mm (2"), 65mm (2 1/2"), 90mm (3 1/2"), 115mm (4 1/2"), 150mm (6"), 215mm (8 1/2"), 305mm (12") and 405mm (16") 4.2.2 Method of Mounting Several common methods of mounting are used. These include: ¨

Stem or direct mounting

¨

Surface mounting

¨

Flush mounting

4.2.3 Location of Connection The pressure connection, in most instances, projects radially from the bottom of the case at the 6 o’clock position, or from the back of the case on the geometrical center, or below the center. Connections are generally provided with a pipe thread of 1/8", 1/4", or 1/2" NPT. Other connections are possible and this will depend on the country and standard that is applicable. 4.2.4 Case and Ring Construction The two broad categories in this classification are cast (or molded) cases and drawn, sheet metal cases. Generally, the lowest cost gauges are made using drawn, sheet metal cases with friction, slip, or threaded rings. Higher quality gauges usually have more sturdy cases of molded plastic or cast metal, with threaded rings, bayonet rings or hinged rings. Cast metal cases are most commonly aluminum or cast iron, and plastic cases are usually molded from a high-impact compound. A safety pattern gauge has a blowout plug or blowout back to relieve in the case of element failure. This type of gauge has a solid wall that separates the dial from the sensing element. Oxygen and acetylene gauges must be safety pattern gauges and generally a safety pattern gauge should be chosen unless the gauge is in low pressure service and a helical bourdon tube is used as the measuring element.


3DG-J34-00001

Page 8 of 28

The lens is usually of shatter proof glass and the dial has black figures on a white background. The pointer zero should be capable of being adjusted without removing the pointer from its shaft. 4.3

ACCURACY CLASSIFICATIONS

The accuracy of an instrument is defined as the difference between the true value (based on a pressure standard) and the indicated value expressed as a percent of the reading or span. % Accuracy of reading = (indicated value-true value)*100/true value % Accuracy of span = (indicated value-true value)*100/span ASME Standard No. B40.100a specifies and grades the accuracy of pressure gauges. In Table 2 the most common commercially available of these grades are listed. Table 2 Grades of Accuracy as Defined in ASME B40.100a (Permissible Error ± % of Span) Accuracy Grade

First 25% of Scale

Middle 50% of Scale

Last 25% of Span

4A

0.1

0.1

0.1

3A

0.25

0.25

0.25

2A

0.5

0.5

0.5

1A

1.0

1.0

1.0

A

2.0

1.0

2.0

B

3.0

2.0

3.0

C

4.0

3.0

4.0

D

5.0

5.0

5.0

When specifying the accuracy for a gauge the application and importance of measurement to the operator should be considered as the cost for the gauge is related to the accuracy specification. Pressure gauge manufacturers frequently refer to the above listed accuracy grades in their technical literature. There are similar equivalent accuracy grades for other international standards. When the ASME standard does not apply then reference to the equivalent national standard for the instrument accuracy grade should be made. Electronic documents, once printed, are uncontrolled and may become outdated Refer to the electronic documents in BecRef for current revisions. Bechtel Confidential © Copyright Bechtel 2002. All rights reserved.

3DG-J34-00001

Page 9 of 28

4.4

SPECIFIC APPLICATIONS

4.4.1 Ammonia Gauges The ammonia gauge has steel or stainless steel internal parts to withstand the corrosive effects of ammonia. It has a dial with two (2) scales; one for pressure and the other for the corresponding temperature of saturated ammonia. Materials such as copper, brass and silver brazing alloys should not be used and the gauge should have the inscription "AMMONIA" OR "NH3" printed on the dial. 4.4.2 Oxygen Gauges Oxygen gauges are fabricated under very high “cleanliness” conditions so that no organic contaminants are present that might react with oxygen and cause an explosion. The gauge must be marked in red on the dial with the inscription "OXYGEN SERVICE - USE NO OIL". 4.4.3 Hydraulic Gauges Hydraulic gauges are specifically constructed for high-pressure service where water or a non corrosive liquid is the pressure medium. They incorporate a special link that is designed to protect the gauge mechanism against a sudden release of pressure. 4.4.4 Acetylene Gauges Acetylene gauges are designed to indicate acetylene pressures and any other gases having similar properties. Materials should be selected suitable for use with the gas pressures being measured and the dial should have the inscription "ACETYLENE" engraved on the front. 4.5

MEASURING ELEMENTS

As briefly mentioned previously, all pressure gauges use some type of elastic element that converts the measured pressure into a proportional motion. Through suitable gears and levers, this motion is transferred into a corresponding motion of the pointer to give an indication of the measured pressure. The degree of accuracy and reliability of such indications depend on the type of measuring element used. Three basic types of measuring elements are commonly used in pressure gauges: ¨

Bourdon elements

¨

Bellows elements

¨

Diaphragm elements

Table 3 covers the pressure ranges normally associated with each of these elements.


3DG-J34-00001

Page 10 of 28

The following is a brief description and principle of operation of each of the elements listed above. Refer to Reference No. 3 or equivalent text book for details pertaining to a detailed description and principle of operation of each of these pressure measuring elements. Bourdon Elements A Bourdon element is the most commonly used element for pressure gauges and is a curved metal tube closed at one end and fixed to a pressure source at the other. Application of pressure at the fixed end results in movement at the free end as the tube cross section deforms. Bourdon tubes come in a C shape, spiral or helix depending on the pressure being measured. Bellows Elements A bellows element expands when pressure is applied to the inside, actuating an indicator, transmitter or controller. Bellows elements are usually used for low pressure less than 10 psig and for vacuum ranges. In pneumatic instruments the bellows usually operates in the range 3 to 15 psig. Diaphragm Elements The diaphragm sensor is a thin flexible metal disc. Pressure to one side of the disc causes a deflection that actuates the indicator, transmitter or controller. Diaphragm elements are used to measure very low pressures and vacuums. One common application is furnace draft pressures. 4.5.1 Low Pressure Applications Diaphragm and capsule type measuring elements are used for pressure sensing for the same amount of pressure change the diaphragm element deflection increases 16 fold if the diameter is doubled. Slack diaphragms are large diameter pressure measuring elements fabricated of synthetic materials like Buna-N or Viton. They are used for very low pressure/draft pressure and differential pressure measurements on boiler furnace, ducts, and HVAC ducts. In many low pressure applications care must be taken to assure that hysteresis of the measuring element does not eventually become a problem, particularly when the sensing/measuring element is installed in a pressure switch and the set point is in the very low pressure range (see Figure 1). If it is anticipated that the low pressure sensing element in a pressure switch may have hysteresis problems then consider using a pressure transmitter and configure the alarm/switch set point in the DCS.


3DG-J34-00001

Page 11 of 28

Table 3 Range for Measuring Elements Element Bourdon

Application Pressure Vacuum Compound

Bellows

Pressure Vacuum Compound

Metallic

Pressure

Diaphragm

Vacuum Compound

4.6

Minimum Span (Commonly Supplied) 0 - 1 bar 0 - 12 psi 0 - 750 mm Hg 0 - 30 in Hg 750 mm Hg - 0 - 1 bar 0 - 30 in Hg - 0 - 15 psi 0 - 25 mm Hg 0 - 1 in Hg 0 - 25 mm Hg 0 - 1 in Hg Any span greater than 25mm (1 in) Hg 0 - 250 mm H2O 0 - 10 in H2O 0 - 250 mm H2O 0 - 10 in H2O Any span greater than 250mm (10 in) H 2O

Maximum Span (Commonly Supplied) 0 - 4000 bar 0 - 60,000 psi 0-750 mm Hg 0-30 in. Hg 0 - 750 mm Hg - 0 - 16 bar 0 - 30 in Hg - 0 - 300 psi 0 - 6 bar 0 - 100 psi 0 - 750 mm Hg 0 - 30 in Hg Any total span less than 6 bar (100 psi) 0 - 0.8 bar 0 - 10 psi 0-750 mm Hg 0-30 in Hg 0 - 750 mm Hg - 0 - 0.8 bar 0 - 30 in Hg - 0 - 10 psi

PRESSURE GAUGE ACCESSORIES

There are many devices available that maybe used in conjunction with pressure gauges to improve their ability to withstand adverse environmental conditions and to broaden their usefulness. Listed below, and briefly described in subsequent paragraphs, are some of these accessories, which are sometimes referred to by the generic term gauge savers. ¨

Diaphragm Seals

¨

Pulsation Dampers/Pressure Snubbers

¨

Siphons

4.6.1 Diaphragm Seals Diaphragm seals should be used under the following circumstances: ¨

Pressure medium would corrode socket and/or sensing element.

¨

Pressure medium might freeze or solidify inside the pressure gauge. Electronic documents, once printed, are uncontrolled and may become outdated Refer to the electronic documents in BecRef for current revisions. Bechtel Confidential © Copyright Bechtel 2002. All rights reserved.

3DG-J34-00001

Page 12 of 28

¨

Pressure medium is highly viscous or a slurry that would erode and/or plug the sensing element.

¨

Pressure medium is a hazardous (toxic and/or radioactive).

¨

High temperature process steams that exceed the maximum temperature rating of the instrument.

Capillary tubing and a diaphragm seal can also be used with pressure transmitters. In vibrating service diaphragm seals should be remotely mounted. Some applications require that the lower housing of the diaphragm seal be welded to the process connection. Under this circumstance the lower housing material of construction must be suitable for welding to the process connection. The diaphragm and exposed housing or bottom part of seals shall be of materials of construction as required by the process. Process connection of bottom bowl shall be 1/2" minimum, except on slurries that shall be 1" minimum, but is usually 1 1/2" or 2". Filling material in diaphragm seals should be compatible with the process fluid. Diaphragm seals shall have a clean out plug (flushing connection) on the process side. 4.6.2 Pulsation Dampers/Pressure Snubbers Rapidly pulsating pressure and short-term pressure surges will produce abnormal wear on the movement bearings and gear teeth and rapidly destroy the accuracy of the gauge. In addition, the indicating pointer may oscillate so rapidly that taking an accurate reading is impossible and, if the pulsation is a large percentage of the span, the bourdon will be subject to early fatigue failure. It is the function of pulsation dampers (or dampeners) to eliminate or at least reduce these adverse effects. Pulsation dampers are available in many forms, but all operate on the principle of restricting the rate of fluid flow into the pressure element, and therefore, the rate at which the pressure can change. Several of these devices are briefly described below. ¨

A “threaded check” is installed in the pressure port of the gauge.

¨

A needle valve, installed between the gauge and the pressure source, is an excellent pulsation dampener. This valve can also act as the block and/or isolation valve.

¨

A snubber using a metal disc available with different grades of porosity.


3DG-J34-00001

Page 13 of 28

4.6.3 Siphons A siphon (a pipe coiled in the shape of a pigs tail) is a heat exchanger (ambient air cools the process fluid) that serves two functions: ¨

Prevent steam or other condensable vapors from entering the pressure gauge. This is desirable because, for example, saturated steam at 100 psig pressure is at a temperature of 337ºF. At that temperature, well above the normally maximum recommended temperature of 120ºF for pressure gauges, the soldered joints of the pressure element (usually a bourdon tube) will fail.

¨

Trap the condensate from draining away, thus effecting a seal through which incoming vapor must pass.

There are more modern versions of the old style “pigtail” siphon which provides a thermal barrier between hot vapors and the pressure measuring device. 4.6.4 Case Pressure Relief Devices On rare occasions the pressure element (bourdon, diaphragm, bellows) of a pressure gauge will rupture. Under this unlikely circumstance it is important to assure that the pressure element rupture will not cause a subsequent pressure gauge enclosure rupture. Pressure gauge manufacturers usually “build-in” some standard method for providing case pressure relief into their gauges. However, as a precaution, the engineer should specify that a means for providing case pressure relief is available. Probably the most common case pressure relief method is to provide a large hole on the back of the pressure gauge enclosure. Since large openings are generally objectionable unless covered or closed in some way, loose fitting grommets are used to cover these holes. The grommet will be ejected due to internal case pressure buildup. Care must be taken that ejection of the grommet is not obstructed by the pressure gauge installation arrangement or that the grommet is “painted shut”. 4.6.5 Over Range Protection In some cases it may be required to protect the gauge from overpressure in excess of the gauge over range. Two common methods are overload stop and gauge protector . The gauge protector is essentially a relief unit that normally allows the gauge to see the line pressure and isolates the gauge from the line pressure when the process pressure exceeds a maximum.


3DG-J34-00001

Page 14 of 28

4.7

LIQUID-FILLED PRESSURE GAUGES

“Liquid filled” pressure gauges are pressure indicators having enclosures that are sealed and filled with a clear viscous fluid. They are not to be confused with pressure gauges with diaphragm seals in which the sensing element, usually a bourdon tube, is filled with a viscous fluid. The use of liquid filled gauges has greatly increased in recent years. They should be considered for use for the below listed reasons: ¨

The fluid will dampen destructive rapid motion of the internal gauge components caused by pressure pulsations, mechanical shock, and vibration.

¨

The fluid will protect the internal gauge components from corrosion damage, an important consideration for service in locations where the general environment is corrosive.

¨

The fluid will act as a lubricant, thus increasing the life of the pressure gauge

¨

If the gauge is located in an area subject to internal condensation, liquid fill will ensure readability at all times

4.8

SELECTION OF PRESSURE GAUGES & TRANSMITTERS

Although this section pertains to the selection criteria for pressure gauges it also applies to a large extent to the selection process of pressure transmitters. (Refer to Section 5.0 for description of pressure transmitters.) When specifying a pressure gauge (or transmitter) the following factors must be taken Into consideration: The selection of a pressure gauge for any given application is a relatively easy process. When specifying a pressure gauge the following factors must be taken into consideration: ¨

Physical and chemical properties of the pressure medium

¨

Temperature of the pressure medium

¨

Pressure range to be measured

¨

Accuracy requirement

¨

Maximum pressure due to upset condition

Each of these factors is considered in the following paragraphs, and, equipped with this and the other information on gauge components and accessories contained in other chapters of this design guide, the user should be able to select a gauge for a specific application that will operate properly with minimum maintenance. Electronic documents, once printed, are uncontrolled and may become outdated Refer to the electronic documents in BecRef for current revisions. Bechtel Confidential © Copyright Bechtel 2002. All rights reserved.

3DG-J34-00001

Page 15 of 28

4.8.1 Nature of the Pressurized Medium Because the measuring element (elastic chamber) of a pressure gauge is exposed directly to the pressurized medium, needless troubles can be avoided by being knowledgeable about the nature of the medium. It may be corrosive, it may solidify at normal atmospheric temperatures, or it may contain tars or solids that will leave deposits inside the elastic chamber. Such conditions can impair the performance of the gauge or make it completely inoperative. The pressure-containing envelope must be corrosion-resistant with respect to the pressurized medium. Published tables listing the corrosion compatibility between various chemicals and materials of construction should be consulted in order to specify and use the most suitable measuring element material. If none of the standard materials is suitable, a diaphragm seal should be used in order to obtain the required corrosion resistance. (Refer to Section 4.6.1) 4.8.2 Temperature of the Pressure Medium The measuring element in a pressure gauge is sometimes fabricated with copper alloy materials that are soldered together. This solder cannot withstand sustained temperatures above 150ºF. Therefore, the temperature of the measured medium inside the bourdon tube should not exceed 120ºF, thus providing a 30ºF margin of safety. If this cannot be accomplished, a siphon described in detail in Section 4.6.3, should be installed between the process and the pressure gauge. If a siphon or other means of cooling cannot be used, some Manufacturers will fabricate pressure gauges with silver brazed or welded joints (the preferred method). These gauges are capable of withstanding temperatures above 150ºF to an absolute maximum of 250ºF. However, once 120ºF is exceeded other problems such as range and zero shift exist. Therefore, only under extraordinary circumstances should a pressure gauge itself be allowed to reach temperatures above 120ºF. 4.8.3 Pressure Range to be Measured Probably the most common application mistake is to specify a pressure gauge range that is much too high for the intended service. The magnitude of the pressure will determine the pressure range selected. The ASME standard B40.100a recommends that the range of a pressure gauge be selected so that the operating pressure occurs in the middle half of the scale, that is, between 25 and 75% of the span. As a guide, the full-scale pressure of the gauge should be twice the operating pressure. By doing so, fatigue life will be improved and a margin provided in the event the operating pressure exceeds its intended value. Occasional application of pressure up to the maximum range of the gauge will not be detrimental. Under most circumstances the Manufacturer’s range that is based on the ASME standard should be selected. Special ranges are costly. Typical ASME standard ranges are listed in Table 4.


3DG-J34-00001

Page 16 of 28

Table 4 ASME B40.100a Standard Pressure Ranges psi

kPa

bar

0/3, 0/5, 0/10, 0/15, 0/30, 0/60, 0/100, 0/160, 0/200, 0/300, 0/600, 0/800, 0/1000, 0/1500, 0/2000, 0/3000, 0/4000, 0/5000, 0/6000, 0/10,000, 0/15000, 0/20000, 0/30000, 0/40000, 0/60000, 0/80000, 0/100000

0/1, 0/1.6, 0/2.5, 0/4, 0/6, 0/10, 0/16, 0/25, 0/40, 0/60, 0/100, 0/160, 0/250, 0/400, 0/600, 0/1000, 0/1600, 0/2500, 0/4000, 0/6000, 0/10000, 0/16000, 0/25000, 0/40000, 0/60000, 0/100000, 0/160000, 0/250000, 0/400000, 0/600000

0/0.01, 0/0.016, 0/0.025, 0/0.04, 0/0.06, 0.1, 0.16, 0/0.25, 0/0.4, 0/0.6, 0/1.0, 0/1.6, 0.2.5, 0/4.0, 0/6.0, 0.10, 0.16, 0/25, 0.40, 0/60, 0/100, 0/160, 0/250, 0/400, 0/600, 0/1000, 0/1600, 0/2500, 0/4000, 0/6000

4.8.4 Accuracy Requirement Under normal circumstances most pressure gauges are not required to be very accurate. Pressure measurements that are required for trending or control purposes are better provided by pressure transmitters (not pressure indicators/gauges) that have a much higher accuracy. Most applications require that the pressure gauge be reliable and sturdy. Unfortunately, pressure gauges - like many other instrumentation devices - tend to be more fragile as their accuracy increases. Thus, the specifying engineer must balance the requirement of installing a sturdy/reliable device with the wish to have this pressure gauge to be as accurate as possible. ASME B40.100a lists the various grades of accuracy. Refer to Table 2 for an excerpt from ASME. In Table 2 the accuracy for various grades of pressure gauges are listed. Most pressure gauges installed are to ASME B40.100a grade 2A, having an accuracy of ± 0.5 % of span. Most “receiver gauges” (see Section 4.1.7) are also ASME B40.100a grade 2A. Most pressure gauges used for calibration purposes are ASME B40.100a grade 3A, ±0.25% of span. On the other hand, the instrument engineer should be aware that pressure gauges furnished as an accessory with a “package unit” frequently are the cheapest the package vendor can obtain. These are sometimes called “Commercial Gauges”, and they have ASME B40.100a grade B accuracy, ± 3/2/3% of span. This means that the first and fourth 25% of span have ±3% accuracy while the second and third 25% (middle 50%) span has an accuracy of ± 2%. 4.9

DIFFERENTIAL PRESSURE GAUGES

Differential Pressure measurements are not only used to measure the pressure drop across various in-line components such as filters, screens, etc., but their most frequent application is in the inferential measurement of level and flow rate. Almost all differential pressure Electronic documents, once printed, are uncontrolled and may become outdated Refer to the electronic documents in BecRef for current revisions. Bechtel Confidential © Copyright Bechtel 2002. All rights reserved.

3DG-J34-00001

Page 17 of 28

gauges/indicators are so-called “motion balance” sensors that employ two bellows. The basic components of the unit are the high and low pressure chambers, the range spring, and the drive assembly to transfer bellows motion to the readout pointer. The bellows in both chambers and the passage between them is liquid-filled. When the unit is installed, the pressure in the high pressure chamber compresses the bellows, so that liquid flows from it into the low side bellows. When the low pressure (or range) bellows expand, they exert a force against the range spring, which determines the span of the instrument. The linear motion of the “range” bellows moves the drive lever, mechanically transmitting a rotary motion through the sealed torque tube assembly to the indicator. The bellows differential gauge is usually used where the process being measured has a very high static pressure and the measured differential pressure is very low. It is possible for higher range differentials to use bourdon tubes (see the Ashcroft range of differential pressure gauges). 4.10

APPLICATION NOTES

As a minimum, pressure gauges are normally installed at the locations listed below. Additional gauges shall be installed if required for the proper operation of a unit. ¨

On each pressure or vacuum vessel.

¨

On the discharge of each pump, compressor and blower, and located inside of any block or check valve.

¨

On the suction and discharge of each stage of a multistage reciprocating compressor.

¨

Across in-line filter for delta P indication.

¨

On the controlled side of all pressure regulating devices.

¨

As tell-tales on rupture disc/relief valve combinations.

¨

At each field pressure switch or blind pressure transmitter.

Normally pressure gauges should be 4-1/2 inch diameter, 1/2 inch NPT bottom connection, molded plastic case, white laminated phenol dials with black graduations. Process gauges should be solid front with blowout back. Movement may be either rotary geared stainless steel or cam and roller type. Accuracy is normally 0.5% of full scale. (ASME B40.100a, Grade 2A) Bourdon tube, socket, and tip shall be stainless steel or shall be made of suitable alloys for specific services. For pressure less than 1 barg (15 psig), and for differential pressure service, the sensing unit may be a bellows, bellows displacement type mercury-less manometer or force balance manometer. Force balance manometers should not be used in pulsating service. Slack diaphragms should also be considered. (refer Section 4.5.1).


3DG-J34-00001

Page 18 of 28

Pulsation dampeners should be considered on all pulsating services such as reciprocating pumps and compressors. For other applications where severe service from pulsing pressure is anticipated, pulsation dampeners may be specified at the instrument engineer’s discretion. Dampening should be provided by either pulsation dampeners or liquid filled gauge bodies with the latter being preferred. (Reference 4.6.2 and 4.7). Steam coil “pigtails” or siphons should be used on all gauges in steam and condensable vapor service so as to maintain temperature of gauge below 120ºF. (Reference 4.6.3).

5.0

PRESSURE TRANSMITTERS

Pressure transmitters are devices that measure and then transmit a signal which is proportional to the pressure sensed/measured to other locations. Most pressure transmitters presently installed in a process plant are the electronic Smart type. However, there are some applications where a pneumatic pressure transmitter is more advantageous to use than an electronic transmitter. For example at an unmanned platform where there is no power and the process gas can be used for instrument air. The most important and fundamental principle of pressure measurement is: Every pressure measurement is actually a differential Pressure measurement. Namely: Gauge Pressure

=

Measured Pressure - Atmospheric Pressure.

Absolute Pressure

=

Measured Pressure - Vacuum.

Differential Pressure=

Measured Pressure #1 - Measured Pressure #2.

Therefore, gauge, absolute, and differential pressure transmitters function according to the same operating principle, and they actually have the same physical appearance. 5.1

ELECTRICAL/ELECTRONIC PRESSURE TRANSMITTERS

There are a number of standards for a signal transmission system, however the most widely accepted and implemented is 4-20 mA dc. Many different methods of obtaining the transmission signal from a pressure transmitter are presently employed. For example: ¨

Capacitance

¨

Strain-Gage

There are also many other measurement principles employed in commercially available pressure transmitters. Among these are LVDT (Linear Variable Differential Transformer), Electronic documents, once printed, are uncontrolled and may become outdated Refer to the electronic documents in BecRef for current revisions. Bechtel Confidential © Copyright Bechtel 2002. All rights reserved.

3DG-J34-00001

Page 19 of 28

reluctance, potentiometer, and rheostat. These methods are normally not employed in the power/process industries. For principles of operation of these transmitters consult the literature of their manufacturers. 5.1.1 Smart Pressure Transmitters Currently, the transmitter of choice is the Smart microprocessor-based type. They are only slightly more expensive than the non-Smart type but offer the advantage of simplified field calibration and an increase in accuracy to +/-0.1%. Smart transmitters contain a microprocessor that can improve sensor performance and/or access to remote communications through a hand-held interface device, control system, or both. The microprocessor improves sensor performance in two ways. First, it can store input/output curves to compensate for sensor output errors caused by factors outside the process, such as ambient or sensor temperature. Second, it can perform math calculations that condition sensor output. A simple software selection, for example, allows users to convert differential pressure transmitters from square root to linear output. The microprocessor can also improve the performance of a pressure transmitter by characterization of its calibration curve. Smart transmitters are currently available using proprietary manufacturers standards or the more widely accepted use both analog and digital protocol as follows: ¨

4 - 20 mA dc (HART)

¨

Profibus

¨

Foundation Fieldbus

5.1.2 Wiring of Pressure Transmitters As mentioned previously, the standard electrical transmission signal is 4-20 mA dc. This signal is generated by the pressure transmitter in a 2-wire configuration. In the 2-wire configuration, the power supply is connected serially to the process loop. As the process pressure varies, it changes the output current. The transmitter is calibrated to give a 4-20 mA dc signal corresponding to the full span of pressure measurement. The fieldbus transmitters can be wired in a variety of different arrangements including bus with spur topology, daisy chain topology and tree topology. The preferred arrangement is using a tree topology and wiring the digital transmitters through a single junction box. Manufacturers are promoting special fieldbus junction boxes that have all the required termination and end of line terminators pre assembled and wired. The power supply for the two wire fieldbus system is located in the segment. The number of transmitters per segment will vary depending on whether intrinsically safe or explosion proof design is adopted. Electronic documents, once printed, are uncontrolled and may become outdated Refer to the electronic documents in BecRef for current revisions. Bechtel Confidential © Copyright Bechtel 2002. All rights reserved.

3DG-J34-00001

Page 20 of 28

5.1.3 Span and Zero Adjustments of Pressure Transmitters The zero and span of modern electronic pressure transmitters can be adjusted very easily. This is particularly helpful when a differential pressure transmitter is used to measure tank level. This feature is described in detail in Design Guide 3DG-J33-0001: “Level Measurement”. 5.2

PNEUMATIC PRESSURE TRANSMITTERS

The standard pneumatic output signal is 3-15 psig. In order for the transmitter to be able to provide this signal, it must be supplied with instrument air having a pressure of at least 20 psig. A filter-regulator combination should be installed in the instrument air supply line, immediately upstream of the transmitter. The transmitted pneumatic output signal can be indicated on a receiver type pressure gauge as described in Section 4.1.6 of this design guide. Consult the literature of a pneumatic pressure transmitter Manufacturers for details regarding their principal of operation. 5.3

APPLICATION NOTES

Pressure and differential pressure transmitters are normally solid state or microprocessor based electronic two-wire transmitters powered by the receiving system. Transmitters can be supplied with either analog or digital integral or separate local indicators depending on the type of transmitter. Where separate indicators are provided these are usually located at a control valve bypass and used to monitor the process pressure when the bypass valve is being used for manual control. Integral local indicators make the requirement for a local pressure gauge next to the transmitter redundant. Local pressure gauges should be limited to applications where positive local pressure indication is required (see Section 4.10). Local indicators should be scaled in engineering units for digital transmitters because of the ease with which range changes can be made. W hen using an analog transmitter select a range of 0 to 100% and use a scale factor, which can easily be changed if the transmitter calibrated range has to be modified. Transmitters must be suitable for and certified for the electrical area classification and should have weatherproof cases to NEMA 4X or IP 65 as a minimum. Transmission signals are 4-20 mA dc for electronic transmitters, 4-20mA dc with digital overlay for HART and digital for fieldbus. Ranges normally start at zero but elevated narrow span ranges are to be used where greater control accuracy is required. Standard accuracy is normally ± 0.1% of calibrated span. Body and element should be a minimum of 316 stainless steel for pressure and differential pressure transmitters. Electronic documents, once printed, are uncontrolled and may become outdated Refer to the electronic documents in BecRef for current revisions. Bechtel Confidential © Copyright Bechtel 2002. All rights reserved.

3DG-J34-00001

Page 21 of 28

Smart transmitters should be considered, or prohibited by industry regulatory authorities for example in the nuclear industry. Smart transmitters are microprocessor-based instruments and come in a variety of industry protocols, HART, proprietary manufacturers protocol, Profibus and Foundation Fieldbus. When using fieldbus transmitters make sure that the device is compatible with the Host. In the case of Foundation Fieldbus, transmitters should have a FF tickmark approval. Process connections should be at least 1/2" NPT and instrument should be equipped with a socket or yoke for mounting on a 2" pipe. A yoke is not required if instrument is mounted on a vessel or process line. In Europe the connections are G1/2"B which is a parallel thread to ISO 228-1. Electrical connections should be 1/2" NPT or 20 mm ISO metric depending on the requirement for the project. When required by the physical properties of the process fluid, a factory sealed filled system with diaphragm shall be selected. If a capillary is used it shall be protected with 316 SS armor. Differential pressure transmitters shall be rated for maximum static pressure of the process and shall be able to withstand over range pressure equal to the meter body rating. 6.0

PRESSURE SWITCHES

When a measured pressure reaches a pre-determined maximum or minimum, it is frequently desirable to turn on an annunciator light, sound an alarm, start or stop a pump, or open/close a valve. These actions can be provided by means of pressure switch. A pressure switch employs the same elastic measuring elements (bourdon, bellows, diaphragm, etc) used in pressure gauges described previously in this design guide. Electric switching assemblies are attached to the measuring element. Both indicating and blind pressure switches are available. The current and voltage rating of pressure switches must be carefully considered. If the current carrying capacity of a certain switch is not large enough for the device to be controlled (e.g. a large heater) the switching circuit should include an auxiliary relay/interposing relay/contactor that does have sufficient current-carrying capacity.


3DG-J34-00001

Page 22 of 28

Pressure switches have a limited life-time. The measuring element’s metal can fatigue and arcing, burning, and chattering will age the electrical contacts. Users and Manufacturers of pressure switches have standardized on the following pressure switch terminology: ¨

The adjustable range is the pressure range within which the actuation point (set point) can be adjusted.

¨

The set point is that pressure which actuates the switch to open or close the electrical circuit (depending on how the switch is wired). It may actuate either on increasing or decreasing measurement at that point. Set point accuracy defines the ability of a pressure switch to operate repetitively at its set point.

¨

The dead band or differential is the difference between the set point and the reactivation point. If the switch set point and the normal pressure are close together and the dead band is to large then this can prevent the switch from resetting when the pressure returns to normal.

¨

Tolerance is usually referred to as the repeatable accuracy of the reactivation point.

These definitions are shown schematically on Figure 1. Pressure switches should be selected to allow accurate process setting and capability to withstand maximum upset pressure. The engineer must also ascertain that the pressure switch meets hysteresis and repeatability requirements. Pressure switch set point should be in the middle third of the range. Pressure switches should be DPDT (Double Pole Double Throw) with internally adjustable set point. A DPDT switch provides more options and flexibility than a SPDT (Single Pole Double Throw) switch. Switch contacts should be hermetically sealed and rated for the circuit service or operated device. For Intrinsically Safe and low current circuits the switch contacts should be plated with a noble metal such as gold or silver to minimize oxidation. Pressure switches shall be furnished with terminal strips. Minimum rating should be NEMA 4 (IP 65) with other classifications as required. It is now becoming more popular to use a transmitter instead of a switch. Transmitters offers advantages such as wider pressure ranges, easier set point changes, available diagnostics (smart transmitters), failure indication, no need for separate junction boxes and separate multicores. Standardizing on transmitters also reduces spares inventories.


3DG-J34-00001

Page 23 of 28

Figure 1 Pressure Switch Terminology

7.0

PRESSURE INSTRUMENT INSTALLATION

Pressure measuring devices such as gauges, transmitters and switches must be connected to and isolated from the process line. For correct measurement of the line pressure it is essential that good instrument engineering principles are followed: Instrument Isolation Pressure tapping points on the line should be on the top for horizontal lines and on the side for vertical lines and provided with a suitable piping isolation valve (root valve), generally 1/2" or 3/4" is provided. The root valve is supplied by the Piping discipline and it is advisable to agree at the beginning of a project with the Piping discipline the type and size of connection that is provided at this valve so that the instrument hook-up matches. In addition the pressure instrument is fitted with an instrument valve(s) or manifold valve for isolation, draining or venting. Installation General Wherever possible pressure instruments should be located as close to the tapping point as possible unless temperature or vibration prohibits this. Direct mounting reduces installation and connections, which are potential sources of leakage. Generally the impulse lines are run Electronic documents, once printed, are uncontrolled and may become outdated Refer to the electronic documents in BecRef for current revisions. Bechtel Confidential © Copyright Bechtel 2002. All rights reserved.

3DG-J34-00001

Page 24 of 28

in tube however for very high pressure (~ANSI 2500#) pipe is popular, particularly if the piping specification is a welded specification where threaded connections are prohibited. When using tube or pipe for instrument hookups it is important to ensure that the thickness of the tube and schedule of the pipe is suitable for the design pressures of the piping system. In gas service always mount the instrument above the tapping point and slope the impulse line so that it drains back the main process line. In liquid, steam and condensable vapor service mount the instrument below the tapping point so that the impulse line is full of the process fluid. In low pressure applications the height of the liquid column has to account for in the calibration of the instrument. With high viscosity liquids use a capillary of short a length as possible.

8.0

WORK PROCESS

8.1

PROCESS DATA

Process data is the responsibility of the Process or Mechanical engineer depending on the office and business unit. The Instrument/control engineer needs the process data to specify the pressure instrument. Information required from Process/Mechanical includes the normal, maximum, design, alarm set pressures and normal/maximum temperatures. 8.2

MATERIALS OF CONSTRUCTION

Generally element materials are stainless steel as a minimum. The materials of construction for the element or wetted parts of the pressure instrument is obtained by reference to the project Piping specification or Material Selection Guide. Exotic materials such as monel and the requirement for NACE (materials for sour service) are selected on the basis of these documents. For hydrogen service special care should be taken in the selection of element materials to prevent permeation, gold plated diaphragms are often used. 8.3

DATASHEETS

Instrument datasheets are required for all pressure instruments. Refer to Design Guide 3DGJ21-00008 for information on datasheets. Datasheets completion generally utilizes a computer system such as InTools. InSpec or CAIES. As a minimum the following data is required to specify the basic types of pressure instruments. Refer to the specific instrument datasheets for full details of specification requirements. Pressure Gauge Tag number Electronic documents, once printed, are uncontrolled and may become outdated Refer to the electronic documents in BecRef for current revisions. Bechtel Confidential © Copyright Bechtel 2002. All rights reserved.

3DG-J34-00001

Page 25 of 28

Service Process fluid Maximum temperature Maximum pressure Normal operating pressure Scale range Element Element material Accuracy Over-range protection Dial size Safety Pattern Accessories, Diaphragm seal, Snubber, Siphon Process connection/rating Pressure Transmitter Tag number Service Process fluid Maximum temperature Maximum pressure Normal operating pressure Scale range Body material Element material Blind/indicating Electronic documents, once printed, are uncontrolled and may become outdated Refer to the electronic documents in BecRef for current revisions. Bechtel Confidential © Copyright Bechtel 2002. All rights reserved.

3DG-J34-00001

Page 26 of 28

Process connection/rating Electrical entry Enclosure type IS or explosion proof Output Pressure Switch Tag number Service Process fluid Maximum temperature Maximum pressure Normal operating pressure Scale range Body material Element material Blind/indicating Process connection/rating Electrical entry Enclosure type IS or explosion proof Switch action Set point high Set point low Contact rating


3DG-J34-00001

Page 27 of 28

9.0

REFERENCES

¨

API RP 551 Pressure Measurement Instrumentation, Section 4 Pressure

¨

ISA Industrial Measurement Series - Pressure

¨

Instrument Engineers Handbook Volume 2 Process Control Bela G Liptak


3DG-J34-00001

Page 28 of 28

3DG J34 Pressure Measurement Test

Recommend Documents