Steam Trap

TRAINING MANUAL – PIPING STEAM TRAP Uhde India Limited

DOC No. : 29040-PI-UFR-0029 Rev.

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CONTENTS

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Cover Sheet

1

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Introduction

2

2.0

Effect of Poor Venting of Steam Network

2–3

3.0

Effect of Condensate Accumulation

3–4

4.0

Steam Trap Types

4 – 18

5.0

Calculating Condensate Loads

18 – 20

Applicable Revision: Prepared:

Checked:

Approved:

Date: First Edition: R0 Prepared: DNL

Date:

Date:

Checked: AKB

Approved: RUD

Date: File Name: C- 29

Date: Server: PUNE: KUMUS 207

Directory: PUNE: Refer \ Pi \ Training Manual

Date: VKO: KUMUS 209

VKO: Refer \ Training Manual


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INTRODUCTION:

Steam is perhaps one of the most efficient mode of energy transportation in the present times in Process Industry. However it has a potential of becoming highly inefficient if certain aspects associated with its handling are not appropriately addressed in the design of steam network. The aspects, which are of particular concern with respect to efficient and safe design of steam network, are: •

The rapidness of evacuation of air or non-condensible gases from the steam network.

•

The efficiency of heat transfer mechanism, which is directly related to the mode of heat transfer (i.e. latent heat or sensible heat)

•

The integrity of system design under the influence of slug forces due the phenomena of Water Hammer, essentially resulting from accumulation of Condensate.

Why a Steam Trap? The answer to all above concerns related to Steam Network design, is obviously an automatic device which can differentiate between the condensate and the air/ non-condensible gases (present in the system during initial startup) from the steam, to be able to selectively and efficiently discharge the air/ non-condensible gases and condensate from the system, at the same time to disallow the steam from escaping from the system. Steam Trap is some such automatic draining device, which satisfies all above functional requirements in terms of discharging the condensate at the required flow rate under varying inlet and back pressure conditions, at the same time allowing a rapid evacuation of cool air/ non-condensible gases from the system.

2.0

EFFECT OF POOR VENTING OF STEAM NETWORK:

Unlike the other piping systems employed in handling liquids, the operating efficiency of the heat transfer equipment using steam as a heating medium is severely hit by the presence of initial air (or any other non-condensible gas) resulting from a poorly vented piping system across the equipment. Consequentially longer than designed time for attaining the required temperature which could slow down the production or some times may even affect the quality of the product due to uneven temperature distribution across the heat exchanger. The graph below (refer Figure-1,Exhibit 34.1) shows the reduction in temperature due to different percentage of air in the steam.

Needless to say that the steam trap employed in such applications must primarily initially be able to vent out the trapped air of the piping system as quickly as possible to disallow the same to get mixed with the steam.



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Figure-1 (Exhibit 34.1) In summary the trapped air/ non-condensible gases must be purged out of the system through an efficient built-in vent provided in the steam trap for following reasons:

3.0

•

On start up the steam can fill into the system only fast as the air is vented out of it.

•

An air steam mixture has a temperature well below the steam temperature (at the corresponding pressure) lowering the heat transfer efficiency.

•

Air is an insulator and clings to the internal surface of the pipe or shell causing slow and uneven heat transfer.

•

The non condensible gases could dissolve in the condensate to form acids (e.g. carbonic acid) which may attach the material of construction of the piping/ equipment

EFFECT OF CONDENSATE ACCUMULATION:

As shown in the sketch below (refer Figure-1, Exhibit 34.2) the condensate accumulating on the bottom surface of the horizontal pipe, is swept along by the velocity of the steam passing over it. As steam approaches to high velocities the condensate gradually tends to form a solid slug of incompressible water, also flowing with almost the same velocity as that of the driving steam. When this water slug is suddenly stopped by any obstruction in the flow passage such as bends, valves, fittings etc there is a sudden loss of momentum that it possessed while in motion , resulting into a pressure wave (surge) which could produce momentary forces of very large magnitude (i.e. water hammer effect) and could cause damage to the piping system.



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S k e tc h sh o w in g p ro g re ss iv e b u ilt u p o f C o n d e n sa te S lu g

Figure-1 (Exhibit 34.2)

A trap is therefore required on typically following locations where Condensate is likely to get formed.

1. Steam Distribution Headers: •

Where self-draining of the condensate is intercepted by vertical risers/ loops.

•

At low points and at approximately every 50 – 60 Mts. Of distance on horizontal pipe.

•

Ahead of all possible Dead Ends (e.g. shutoff valves on bypass lines)

2. Steam Tracing/ Jacketing Service: •

At the steam supply manifolds for feeder lines.

•

At the condensate collection manifolds.

3.

4.0

Steam Operated Equipment:

•

Ahead of Humidifier, Pumps and Turbines etc.

•

Condensate outlet from the Equipment using steam (e.g. Condensers, Coils, Heaters, Drier etc.)

STEAM TRAP TYPES:

Based on their working principle the Steam Trap can be classified as follows: Density Operated: • •

Ball Float Inverted Bucketed



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Temperature Operated (Thermostatic): • •

Balanced Pressure Liquid Expansion

Kinetic Energy Operated: • •

Thermodynamic Impulse or Orifice

4.1 BALL FLOAT TRAP:

The Ball Float Trap works on the principal of sensing the difference in the density of Steam and Condensate. The accompanying sketch below (refer Figure-1, Exhibit 34.3) shows the constructional details of the above Trap.


The Condensate on reaching (and eventually filling) the trap body, causes the Float to rise as per the level of Condensate in the Trap body, consequentially lifting the disc element off its seat to allow the Condensate to exit out. The level of the disc/ seat assembly is so adjusted that it always remains immersed in the Condensate even at the lowest level of Float (i.e. when the flow of Condensate is fully stopped). Therefore the steam can never escape from the exit port. More over the Float responds almost instantly to any build up of Condensate above its minimum level causing the Condensate to exit out of the discharge port as soon as it is formed. With regards to venting of initial air, the Float Traps are normally fitted with an auxiliary vent

(pressure balanced thermostatic trap) which remain wide open (at cold

condition) to allow quick venting of air during startup. However as soon the Steam/ Condensate (following cold air) fills the trap the auxiliary vent on sensing the steam temperature snaps shut to disallow the steam to escape.



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Salient Features: •

Continuous and modulating discharge prevents any pressure fluctuations in the condensate system.

•

High heat transfer efficiency (i.e. condensate is discharged as rapidly as it is formed without any banking up.

•

High air venting capacity through fully open auxiliary vent

•

It can successfully perform for a fairly high range of steam pressure.

•

Large and bulky.

•

Ball float mechanism is susceptible to damage due to Water Hammer.

•

Can easily freeze under subzero ambient conditions and hence must be provided with Steam Lock Release Valve (SLR) feature.

4.2 INVERTED BUCKET TRAP:

The accompanying sketch (refer Figure-1, Exhibit 34.4) below shows the constructional details of an Inverted Bucket Trap. As the name implies the working portion of the Trap consists of an Inverted Bucket attached through a lever mechanism to a valve. A vent hole provided in the flat bottom of the Inverted Bucket is essential for initial venting of the cold air during startup operation. Similarly a water seal must be maintained at the bottom rim of the bucket to prevent blowing of live steam.




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The sequence of working of the trap is shown in the sketch (refer Figure-1, Exhibit 34.5) below


In position 1 the trap body is full of Condensate. The bucket therefore tries to settle under its own weight, pulling down the valve off its seat. Consequentially the condensate is pushed out through the open discharge port.

In Position 2 the mixture of Condensate and Steam enter the trap body. The Condensate settles down at the bottom while the steam occupies the upper space of the Inverted Bucket by displacing the Condensate which, continues to flow through the open discharge port. With more and more steam displacing the Condensate from the bucket, causes the bucket to float up to close the discharge port before the steam could push the Condensate further down to blow the bottom seal and escape through the discharge port. With lapse of time as the steam filled in the upper portion of the Inverted Bucket condenses, the bucket once again gets filled with condensate to gradually settle down under its own weight, causing the cycle from position 1 to repeat.

The venting of cold air during the initial startup is achieved by a tiny hole provided at the top of the Inverted Bucket. On filling of the bucket with cold air (which does not condense unlike steam) the same is allowed to escape through the above hole. The vent hole size can not be made disproportionately large to prevent (rather reduce) the loss of live steam by bubbling through the above hole during normal operation. A practical approach in sizing the vent hole is therefore a compromise between its air venting capacity vis-à-vis the loss of live steam during normal operation of the Trap.



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Limited capacity as far as venting of cold air during start up, is concerned.

•

Must maintain water seal. Loss of water seal will defeat the functioning of the Trap.

•

All Inverted Bucket Traps incur some amount of steam loss (bubbling through the vent hole).

•

The condensate discharge is not continuous. Hence reduced heat transfer efficiency.

•

The above traps are suitable for operating at fairly high pressures.

•

The above traps have a fairly high resistance to water hammer since the condensate/ steam is lead to a reasonably large volume of the bucket.

•

The above traps must be protected from freezing under subzero ambient conditions.

4.3 LIQUID EXPANSION TRAP:

This is one of the simplest design amongst Thermostatic type of Traps. The accompanying sketch (refer Figure-1, Exhibit 34.6) below shows the constructional features of a Liquid Expansion Trap.


It consists of an oil filled flexible thermostatic element which expands when heated by the fluid (i.e. steam/ condensate) surrounding it to exert the necessary amount of force to close the valve disc against the seat. It is possible by altering the gap between the disc and the seat (by adjusting the spring fitted at the end of the thermostatic element), to effectively alter the maximum temperature of the fluid (condensate) which is allowed to discharge through the seat.



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That means that the moment the fluid entering the trap body is higher than the set temperature it will be stopped by the flexible thermostatic element (i.e. by closing the disc). Depending upon the anticipated condensation temperature of the steam, the above setting could be initially adjusted to allow the discharge of the condensate just a little below the steam temperature. The problem with the above trap is that the discharge temperature of the condensate could be adjusted initially only once and the Trap as such is incapable of self adjusting to the varying steam pressure (hence temperature).

The above phenomena can be easily explained in the sketch (refer Figure-1, Exhibit 34.7) below showing the saturation curve of the steam. Assuming that the Trap is set to discharge the Condensate at temperature slightly below T1 (i.e. the saturation temperature of steam at pressure P1), it will operate satisfactorily as long as the operating steam pressure remains close to P1. However the same trap on seeing a higher steam pressure P2 which would result into Condensate formation at temperature T2 (i.e. well above T1) will allow the Condensate to discharge only when it is cooled down to a temperature close to (below) T1, resulting into serious water logging problem. Similarly if the trap sees steam at lower pressure P3 it will allow blowing of live steam.




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Since the Condensate discharge temperature can be manipulated, the above trap allows attaining high thermal efficiency (i.e. by utilizing the latent heat of steam as well sensible heat of Condensate).

•

In terms of heat transfer efficiency the above trap is a poor performer.

•

Does not require protection against freezing due to low ambient temperature when connected through a free draining discharge system.

•

Normally lower Condensate discharge temperature eliminates concerns of flashing steam.

•

The use is limited to the applications where banking of condensate can be accepted.

•

It can not self adjust to varying steam pressure conditions.

•

The Bronze Bellows of the thermostatic element may become corroded when handling corrosive Condensate.

4.4 BALANCED PRESSURE TRAP:

Balanced Pressure is an improvement on the Liquid Expansion Thermostatic Trap in the sense that the above Trap is self adjusting to varying operating steam pressure conditions within its designed pressure range. That means that the above trap is capable of discharging the Condensate with small amount of sub-cooling (irrespective of steam pressure), at the same time it does not allow blowing of live steam.

The accompanying sketch (refer Figure-1, Exhibit 34.8) below shows the schematic arrangement of the constructional details of a Pressure Balanced Bellow. The thermostatic flexible element (as in case of Liquid Expansion Trap) is replaced with a suitably designed Bellow, which is sensitive enough to respond to the internal and external pressures acting on it. The above Bellow instead of being filled with oil is evacuated and then filled with a small quantity of some sort of liquid, which has a boiling point somewhat lower than water. This allows the filled liquid (on seeing the steam) to boil off and generate an internal vapour pressure in excess of the steam pressure surrounding it, by an amount required for expanding the bellow to close the flow passage. That means that the live steam filling the trap, must cool down to form a sub-cooled Condensate (a few degrees below steam condensation temperature) to cause the internal pressure of the bellow to diminish and thereby allowing passage for the Condensate through the discharge port.



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Since the above functioning principle is based on the differential pressure (i.e. the difference between the internal and external pressure applied on the bellow) it is independent of the operating pressure of the steam. The above Trap is therefore reasonably responsive to the varying steam pressure conditions. Due to the thin wall of the Bellow element, it is able to respond to temperature changes (due to varying steam pressures) fairly quickly resulting into the response line shown in the curve shown below (refer Figure-1, Exhibit 34.9). The above trap is also fairly quick with regards to free venting of cool air during initial startup, due to the unobstructed passage of flow in the shrunk position of the Bellow.





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High air venting capacity for startup.

•

Self adjusting, to function satisfactorily under varying steam pressure (within its operating pressure range) conditions without any external adjustments.

•

Does not require freezing protection under subzero ambient conditions when its discharge is piped as a free draining line.

•

Simple and compact design for large capacity Trap.

•

The Trap is generally not suitable for super heated steam

•

Limited resistance to water hammer.

•

Some amount of sub-cooling of condensate can not be avoided.

•

Susceptible for corrosion attack, when handling corrosive condensate.

4.5 BIMETALLIC TRAP:

The Bimetallic Trap is by and large the most robust design amongst the various types of Traps in common use. It utilizes bimetallic strips of dissimilar metals stacked together in pairs. The accompanying sketch (refer Figure-1, Exhibit 34.10) below shows the schematic constructional details of the above Trap.


On sensing the temperature of the steam/ condensate, owing to dissimilar growth of bimetal, the above strips tend to warp and effectively push the disc down against the seat to stop the flow of fluid through the discharge port. Similar to Liquid Expansion Trap it is possible in a Bimetallic Trap, to set the discharge temperature of the Condensate by adjusting the position of the lock nut at the top of the deck of the bimetallic strips.



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The response line of the trap in the above said design is also similar to Liquid Expansion Trap (i.e. as shown in Figure-1, Exhibit 34.7) which has its own disadvantages as explained earlier.

An improved version of the above design is possible by utilizing an inverted disc valve and the bimetallic strips arranged in a reverse manner as shown in the sketch (refer Figure-1, Exhibit 34.11) below


In such a design the warping of the strips will exert an upward force to pull the disc up against the seat to close the flow passage. The net closure force however being the difference between the axial upward pull (due to warping of strips at temperature) and the downward push due to the line pressure acting on the disc. As a result the response line of the Trap shows some improvement as shown in the sketch (refer Figure-1, Exhibit 34.12) below.




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A further improvement in the response line of the above Trap is possible by utilizing a modified design of the bimetallic element with multiple prongs as shown in the sketch (refer Figure-1, Exhibit 34.13) below.


The clearance and the contour of the above prongs is so manipulated that each pair of it comes into action at different temperature (corresponding to different steam pressure conditions) to result into the required net upward pull of the disc to close the flow passage. Thereby achieving a response line reasonably close to the steam saturation curve as shown in the sketch (refer Figure-1, Exhibit 34.14)) below.




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With the above design principle in mind, one may be tempted to believe that it is possible adjust the settings of the Trap to bring its response line almost approaching to the saturation curve of the steam. However aspects such as Hysteresis of bimetallic strips may pose practical limitations in achieving the same. Similarly when working with condensate collection systems subject back pressures, a variation in the back pressure may upset the initial settings and it is likely that an increase in the back pressure will cause blowing of live steam through the Trap. The above is demonstrated in the Trap Response Curves in the accompanying sketch (refer Figure-1, Exhibit 34.15 and Figure-1, Exhibit –34.16) below.



It is therefore practical to set the trap response line slightly lower than the steam saturation curve to allow the Condensate discharge with some amount of sub-cooling.



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Fairly robust and compact design

•

Fairly high air venting capacity.

•

Fairly high resistance to Water Hammer.

•

Reasonably good performance against super heated steam.

•

Some amount of sub-cooling of condensate can not be avoided. Modest response to temperature variations.

•

Fairly high thermal efficiency when set to discharge sub-cooled condensate. Possibility of flashing of sub-cooled condensate is minimal.

•

Does not require freezing protection under subzero ambient conditions when its discharge is piped as a free draining line.

•

Bimetal characteristics may change after being in use for some time. Hence may require servicing.

4.6 THERMODYNAMIC TRAP:

This is by far the commonest type of Trap relying on the fact that hot Condensate released under pressure will generate flash steam. The Trap is fairly simple in construction, with only disc as the moving part inside the control chamber fitted on the top of the Trap body. The central inlet port in the body provides a flat seating surface with an annular groove around it to connect it to the discharge port.

Exhibit 34.17



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The functioning of the trap can be understood from the accompanying sketch above. During the initial startup cold air and Condensate lift the disc from the seat to pass radially outwards through the annular groove to the exit port (refer Figure-1, Exhibit 34.17). As the Condensate approaches the accompanying steam temperature it tends to flash giving rise to the velocity of fluid passing under the disc and thereby causing a drop in the pressure. The flashed steam eventually filled in the control chamber, at the same time exerts pressure from above to push the disc down against the seat which snaps the flow passage shut to block the discharge of the condensate (refer Figure-2, Exhibit 34.17). The Disc remains closed under the steam pressure of the flash steam of the control chamber for a while (refer Figure-3, Exhibit 34.17). With lapse of time the flash steam in the control chamber condenses (by loosing heat to the surrounding atmosphere) to once again lift the disc under the upstream line pressure and allow the flow of Condensate.


Compact and light weight design.

•

Owing to simple internal parts it has a fairly high resistance to corrosion.

•

Less prone maintenance.

•

Efficient operation under varying steam pressure and condensate loads.

•

Allows high heat transfer efficiency by discharging condensate at the steam temperature.

•

High resistance to water hammer. As such no condensate banking.

•

Requires a certain minimum inlet pressure (approximately 0.6 bar) to operate (i.e. for lifting the disc)

•

Same model can be used for wide range of operating pressures.

4.7 IMPULSE TRAP:

The Impulse Trap is schematically shown in the accompanying sketch (refer Fifure-1, Exhibit 34.18). It consists of a hollow piston A with a piston disc B working inside a tapered guide Cylinder C.

Figure–1 (Exhibit 34.18)



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At start the main valve (integral part of piston A) is seated against the seat D and the flow takes place through the clearance between disc B and Cylinder C and then through the port E. An increase in the flow of fluid will act on the disc B to lift the piston A off its seat to give an increased flow of condensate through the seat. As the condensate passing between the clearance between disc B and Cylinder C tends to approach the steam temperature it flashes in the upper portion of the space between disc B and piston C. Even though the above flash steam, does bleed through the port E it still builds an intermediate pressure to push the piston A to throttle the flow of condensate through the seat. It is obvious from the above functional description that the Trap is never ever able to provide a tight shut off and will always pass some live steam specially, at light loads. Because of this inherent weakness the above trap is not a preferred choice in the current practice.


Continuous bleed orifice will waste steam at light loads.

•

Close clearances required between various components invariably get choked due to the dirt accompanying the condensate.

•

It is unsuitable for use when the back pressure is in significant proportion to the inlet pressure.

•

Gives rise to water logging. Hence predominantly overwhelmed with the problems of water hammer and corrosion.

•

5.0

The above trap is currently not in common use.

CALCULATING CONDENSATE LOADS:

Very often one may be faced with the situation where precise data regarding the condensate loads may not be available (some times may not be worth while). In such situations the condensate loads can be approximately estimated as per following. O

Condensate Loads/ Hour from the insulated Steam Mains at 21 ambient temperature:

The Table below (refer Figure-1, Exhibit 34.19) provides and estimated condensate load for a 30 Meter of Pipe run

DOC No. : 29040-PI-UFR-0029


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Main Size (NS..MM)

Steam Pressure Bar 50

80

100

150

200

250

0.7

3

4

5

8

10

12

4.0

5

7

9

14

17

21

7.0

6

9

11

16

20

25

20.0

10

15

19

28

35

44

42.0

15

22

28

41

52

64

Steam Tracer Lines:

Approximate load can be considered as 30 Kg/ Hour for each 30 Meters of tracer line.

Heating Water with Steam:

Kg Condensate/ Hour

=

LPH

O

* Temperature rise in C 500

Heating Fuel Oil with Steam

Kg Condensate/ Hour

=

O

LPH * Temperature rise in C 1000

Heating Air with Steam:

Kg Condensate/ Hour

=

3

O

NM / Min * Temperature rise in C 27

Heating Solid Materials (e.g Sterilizers, Autoclaves, Retorts):

Kg Condensate/ Hour

=

O

W * Cp * Temperature rise in C L*t

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Where: W

=

Weight of Material in Kg.

Cp

=

Specific Heat of Material in K Cal./ Kg. C

L

=

Latent Heat of Steam K Cal./ Kg

T

=

Heating Time in Hours

O

Heating Liquid in Steam Jacketed Kettles:

Same as above except that the Specific Heat of Liquid will apply.

Steam Jacketed Dryers:

Kg Condensate/ Hour

=

O

600 (Wi – Wf) + (Wi * Temperature rise in C) L

Where:

Wi

=

Initial Weight of Material in Kg./ Hour

Wf

=

Final Weight of Material in Kg./ Hour

L

=

Latent Heat of Steam K Cal./ Kg

Heating Air with Steam (e.g. Pipe Coil and Radiation):

Kg Condensate/ Hour

=

O

A * U * Differential Temp. C (steam and ambient) L

Where:

A

=

Area of Heating Surface in Sq. Meters

U

=

Heat Transfer Co-eff. K. Cal. / Sq Mt. C

O

(Consider a value of 2 for free convection) L

=

Latent Heat of Steam K. Cal./ Kg

Note: Condensate Load to heat the Equipment shall be added wherever applicable.

Steam Trap

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