Column PSV Sizing

Doc. No.: PDG-MUM-XXX Relief Valve Sizing Philosophy

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PROCESS DESIGN GUIDELINE RELIEF VALVE SIZING PHILOSOPHY FOR COLUMNS

Petrofac Petrofac Engineering India Pvt. Ltd.

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14-June-10

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TABLE OF CONTENTS

1.0 Column ................................................................................................................................. 1.1 Applicable scenarios

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1.2 Scenario description and relief load calculations .............................................................


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1. Column Columns are usually protected by a pressure relief valve (or valves) mounted on the top head or overhead line. This means that a single pressure relief installation is used to protect the tower itself plus all associated equipment. This can include the reflux drum, side stream strippers, overhead condenser, reboiler, and possibly the feed and product exchangers, depending on the piping configuration and location of the control valves. A supplementary pressure relief valve may be provided on the reflux drum but usually only for fire purposes.

1. 1. Applicable scenarios Individual scenarios 1. External fire 2. Blocked vapour outlet 3. Loss of overhead cooling 4. Reflux failure 5. Control valve failure 6. Abnormal heat input 7. Tube rupture within reboiler/condenser 8. Feed failure 9. Overfilling(Blocked liquid out) 10. Accumulation of Non-Condensables 11. Loss of Heat in Series Fractionation Systems 12. Pump around failure Global Scenarios 1. Instrument Air Failure 2. Total Power Failure 3. Partial Power Failure 4. Simultaneous External Fire 5. Cooling Water Failure


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1.2 . Scenario description and relief load calculation procedure: Individual scenarios 1. External fire Steps to be followed for relief load calculations: 1. During fire, all feed and output streams to and from the fire affected equipment and all internal heat sources within the process are assumed to have ceased. 2. The fire load will be determined as per the section. (Refer chapter 3 ).The basis for fire load will be the wetted surface up to 7.6 m (25 ft) or client standard from grade of column. 3. Normal level in bottom plus liquid hold-up from all trays dumped to the normal level in the column bottom. 4. The level in reboiler is to be included if the reboiler is an integral part of the column. 5. To account for piping an additional 10% should be added to the wetted area. Assume additional design margin of 10% on overall wetted area. 2. Blocked vapor outlet A blocked column outlet is a potential overpressure scenario when the following two criteria are met: 1. A mechanism, such as a block valve, exists to block a column outlet. In the event that a system of columns in open communication can be blocked by the action of a single outlet block valve (i.e. on the outlet of the last vessel), the potential for a blocked outlet should be considered for each vessel in the system. 2. A pressure source (pump, compressor, high pressure reservoir, heat etc.) in excess of the column MAWP plus allowable accumulation is present upstream. The maximum pressure for sources can generally be determined as follows:  

The upstream relief device set point plus allowable overpressure. The physical limitations of equipment such as the dead head pressure for centrifugal pumps and compressors. To restate, the upstream pump or compressor must be able to exceed the MAWP plus allowable overpressure (typically 110% of MAWP) in order to be a potential source of overpressure.

The relief load is calculated as follows. 1. The quantity of material (column overhead stream) to be relieved should be determined at conditions that correspond to relieving conditions instead of at normal operating conditions from a maximum flow case. 2. The required relieving rate is often reduced appreciably when this difference in conditions is considered. The effect of frictional-pressure drop in the connecting line between the source of overpressure and the system being protected should also be considered in determining the required relieving rate. 3. The relief load can be taken conservatively the normal flow rate from H&MB or technical flow rate i.e by considering pro rate factor /margin given in process design basis or directly taken from equipment sizing calculations.


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3. Loss of overhead cooling Typically cooling water, air, process fluid acts as a source of cooling for the overhead vapour from the column. Independent scenarios should be defined for loss of every single source. In case of failure of any of this cooling source the column may get overpressurized and the PSV on the column will provide the relief. Loss of overhead cooling causes a pressure build-up due to lack of overhead condensation. Reflux continues until the accumulator liquid level is lost. For loss of reflux or overhead product, no relief is required until the accumulator fills and the condenser floods. Flooding is assumed to occur if there is inadequate surge volume in the overhead accumulator to provide enough time (10 minutes) for operator response before the accumulator fills with liquid. The calculation procedure to determine the required relief rate is slightly different depending on whether the column has a partial or total condenser. The following calculation method is directly applicable to most typical distillation systems. However, due to the large number of distillation arrangements, consideration should always be given to the specifics of the system prior to applying the below methodology. In the event that no credit is taken for vapor flow from the accumulator, the loss of cooling with continuing reflux is identical to a blocked column overhead case. The normal overhead flow rate (set at the relief pressure and new saturation temperature) provides an upper bound on the loss of cooling with continuing reflux or blocked outlet case. The below procedure should result in a smaller relief requirement unless a larger reboiler duty is used in place of the normal operating duty. 1. Obtain the normal feed(s) rate, temperature, and composition. 2. Obtain the normal reflux rate, temperature, and composition. 3. Obtain the maximum expected reboiler duty. Typically, this will be the design duty unless other guidance supersedes or there is reason to believe that the reboiler is being operated beyond design rates. 4. Obtain the normal operating and relief pressures. 5. Mix (mixer in HYSYS) all of the feeds and the reflux (vapor returns from reboilers excluded) together at the normal column operating pressure. 6. Add the maximum expected reboiler (heater in HYSYS). 7. Separate the liquid and vapor effluent from the heater (separator in HYSYS). Note that, in the event that the heater vaporizes all of the feed, the maximum expected reboiler(s) duty was probably set significantly above the operating duty. If the maximum expected reboiler duty is acceptable proceed to step 8. 8. Compare the overhead rate from the separator to the column overhead rate. The separator overhead rate should be slightly larger than the column overhead rate (assuming the maximum reboiler duty is close to the normal operating duty). If so, the data entered is reasonable, so proceed with step 9. If not, reevaluate the input data to determine the cause of the discrepancy. 9. Repeat steps 4, 5, and 6 at the relieving pressure (includes 10% allowable overpressure). 10. One of three things will occur: One, the feed to the separator is all liquid which indicates that relief is not required. Typically, this means the relief pressure is much greater than the operating pressure. 11. Two, the feed to the separator is superheated vapor. Since it is unrealistic to have superheated vapors in the column, the estimate vapor composition and rate at the top of the column can be determined by setting the mixer outlet to be 100% saturated vapor (specify the pressure and a vapor fraction of 1.0 in HYSYS). 12. Three, the feed to the separator is two phase. In this case, the overhead vapor rate from the separator is the estimated vapor rate from the top of the column at the relief pressure.


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13. For total condensers, the vapor rate from the top of the column, as calculated in #8 is equal to the required relief rate as no outlet is present for the vapors (any pressure controller present on the accumulator is assumed to remain closed). 14. For partial condensers, the required relief rate is equal to the vapor rate (mass terms) from the top of the column, as calculated in #8, minus the normal vapor product rate (mass terms) from the accumulator. Note that differences in composition are neglected when subtracting the two rates. 4. Reflux failure Loss of overhead cooling causes a pressure build-up due to lack of overhead condensation. Reflux continues until the accumulator liquid level is lost. For loss of reflux or overhead product, no relief is required until the accumulator fills and the condenser floods. Estimation of the relief load 1. Obtain the normal feed(s) rate, temperature, and composition. 2. Obtain the maximum expected reboiler duty. Typically, this will be the design duty unless other guidance supersedes or there is reason to believe that the reboiler is being operated beyond design rates. 3. Obtain the relief pressure. 4. Based on the normal bottoms composition, the relief pressure and the maximum expected reboiler duty; obtain the effluent conditions from the reboiler. Typically this is accomplished using a heater operation and specifying the duty and mass percent vapor out of the reboiler to obtain the flow and temperature of the reboiler effluent. Note that the mass percent vapor from the reboiler may vary. Kettle type reboilers result in 100% vapor effluent, while thermosyphon and forced circulation reboilers often have a two phase effluent. 5. Using a typical absorber operation input the reboiler effluent stream on the bottom tray and the uppermost feed on the top tray. Any additional feeds should also be input on the appropriate tray. 6. For total condensers, the vapor rate from the top of the absorber, as calculated in #5, is equal to the required relief rate as no outlet is present for the vapors (any pressure controller present on the accumulator is assumed to remain closed). 7. For partial condensers, the required relief rate is equal to the vapor rate (mass terms) from the top of the absorber (as calculated in #5) minus the normal vapor product rate (mass terms) from the accumulator. Note that differences in composition are neglected when subtracting the two rates. When the PSV is found inadequate with the reboiler design duty then the relief load with reduced duty should be estimated and again the adequacy can be checked. The reduced duty is calculated as follows.

Reduced duty calculation a) Finding of UA Model the Column reboiler as exchanger as follows: 1. Cold side inlet: Take process stream to the exchanger at same conditions as going to the reboiler from the column in the base simulation. 2. Hot side inlet: Provide steam to the exchanger at normal conditions. If temperature is not available then use saturation temperature.


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3. Cold side outlet: Set vapour fraction on process outlet side same as that of the outlet fraction from Reboiler going to the column. 4. Hot side outlet: For steam side set vapour fraction equal to zero. 5. Converge the exchanger; the heat duty obtained should be close to the base case simulation duty. 6. The UA value is obtained which is to be used in reduced duty calculations. b) Estimation of reduced duty 1. Again take new exchanger. 2. Cold side inlet: Provide feed stream to it with normal composition and at relief pressure. Make vapour fraction zero to get bubble point temperature. 3. Hot side inlet: use same stream on hot side obtained in first step. 4. Specify the UA obtained earlier for the new exchanger. 5. Converge the exchanger. The reduced duty is obtained. This duty is then used in for finding out relief rate. Do not use reboiler duty that is less than 50% of design, since duty less than this might result in column dumping. Consult client if a calculated reboiler duty less than 50% is to be used. 5. Control valve failure Control system is subject to both individual failure as well as collective failure, if caused by total loss of instrument air. The instrument air failure case is covered under global scenarios. The individual failure scenario takes each control valve individually and determines the relief when the valve is inadvertently full opened and fully closed during normal operation regardless of the failure position. This can be caused by operator error or control loop failure. Applicability of the scenario: Check whether the normal operating pressure upstream of the listed control valve based on the normal operating pressure of upstream fluid exceeds the lowest MAWP/design pressure of the system or not. If no then, overpressure is not expected. For relief load calculations refer section (xxxxx) 6. Abnormal Heat Input from Reboiler In the event of control failure on the heat input to the reboiler, it is possible to generate vapors in excess of the condensing capacity of the system. The unbalanced heat input in this case is calculated based on the maximum reboiler duty. Credit normally can be taken in this case for the additional condenser capacity at elevated pressure; however, since this case is usually not controlling, normal overhead condenser duty is usually used to calculate a conservative load for this case. If the condenser is capable of condensing all vapor generated due to abnormal heat input, this case won ’t create overpressure.

7. Reboiler/Condenser Tube Failure Thermal shock, vibration, and corrosion can cause tube failure in shell and tube heat exchanger. The result is the possibility that the high-pressure stream will overpressure equipment on the low-pressure side of the exchanger. The possible pressure rise must be ascertained to determine whether additional pressure relief would be required if flow from the tube rupture were to discharge into the lower-pressure stream.


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Pressure relief for tube rupture is not required where the low pressure exchanger side (including upstream and downstream systems) is hydrotested at or above the high pressure exchanger side design pressure. In other words, if heat exchanger is designed as per 2/3 rd rule (now 10/13th rule based on ASMEs hydrotest pressure requirement), pressure relief for tube rupture is not required. For relief load calculations refer section (xxxxx)

8. Feed failure Identify number of scenarios depending on number of feed streams to the column. The relief rate will be calculated as follows: 1. For steady state simulation, determine the relief loads using the normal feed rates to the column. Then reduce column feed to 1 kg/h or whatever minimum flow the column simulator will converge with. This may need to be done in small feed reduction steps to avoid convergence problems. 2. Compare the accumulator overhead vapour flow rate (MMSCFD) from this relief case against the normal accumulator overhead flow rate. If the relief case flow rate is higher than the normal case flow rate, then relief rate will be the difference between the two flow rates. 3. If the flow rate is less than the normal case flow rate then the scenario is not applicable.

9. Overfilling (Blocked liquid outlet) Blockage of the bottom stream usually does not have a relief impact, since there is normally a large surge volume in the bottom of columns so that operations has enough time to respond to the event well before relief occurs. In this case check the time required to fill the column and compare it with operator response time (approximately 15 minutes). If high level alarm is present overfilling is not applicable. Also check project philosophy for operator intervention.

10. Accumulation of Non-Condensables Non-condensables do not accumulate under normal conditions since they are released with the process vapor streams. However, with certain piping configurations, it is possible for non-condensables to accumulate to the point that a condenser is blocked. 11. Loss of Heat in Series Fractionation Systems In series fractionation (that is, where the bottoms from the first column feed into the second column, and the bottoms from the second feed into the third), the loss of heat input to a column can cause light ends to enter the bottoms stream from that column and overpressure the following column. Under this condition, the overhead load of the second column may consist of its normal vapor load, plus the light ends from the first column. If this added load cannot be condensed, then the relief will occur from the downstream column. It is also possible that accumulation of non-condensables could result, causing loss of cooling capacity. Relief of the additional light ends could then be additive with the load due to loss of cooling. The procedure to calculate the relief is as follows: 1. This scenario is evaluated using a "Reboiled Absorber" operation in Aspen HYSYS. 2. The feed stream of upstream column are mixed and fed to Reboiled Absorber. 3. The reflux at normal operating flow rate, relief pressure, and saturation temperature (based on the downstream column reflux stream is fed to the column. 4. The combined normal heating duty of the Reboiler and the normal cooling duty of the condenser obtained from the HYSYS simulation file are applied to the column, and the column is specified to operate at the relief pressure.


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If there are no resulting vapors downstream of the condenser then, no relief is expected.

12. Pump around failure Circulation through one pump around circuit at a time is assumed to be completely lost. The required relieving rate is the vaporization rate caused by an amount of heat equal to that removed in the Pump-around circuit. The latent heat of vaporization corresponds to the latent heat under the relieving conditions of temperature and pressure at the point of relief.

Global Scenarios

1. Loss of Instrument Air Instrument air failure may be local or general. Depending on the service, local failure can cause overpressure for various reasons. General failure causes plant shutdown with each control valve reverting to its fail-safe position in conformance with safety and overpressure limitation considerations. The fail-safe characteristics of each control valve are established as an integral part of overall plant design. 2. Total Power Failure Total power failure is assumed to be 100 percent loss of electric power except for the Uninterruptable Power Supply (UPS). The effect of a power failure on all the flows in a unit must be evaluated to establish the relief flows. Backup electric power sources must be considered in the analysis of the effects of electric power failure. For example, critical electronic instrumentation is generally connected to the Uninterruptable Power Supply (UPS) to ensure safe and orderly shutdown upon loss of normal electric power supply. In case of power loss feed fails, side draw fails, pump around fails, bottom product stops, reflux stops, overhead liquid product fails, accumulator overhead vapors stops, however heat input continue via steam reboiler to the liquid inventory in the column. Use normal reboiler duty, bottom product liquid composition and find out the relief load. 3. Partial Power Failure A partial power failure due to the loss of the particular electric circuit would result in a loss of the relevant electrically driven equipment. Depending upon how a group of motors are connected to the power source, multiple failure conditions may occur. For aerial coolers fans – Find out the remaining no of fans expected to be in service. 4. Simultaneous External Fire Find out fire area from plot plan. Indentify the total relief load due to fire from all the equipments in the fire area. This will be total load for simultaneous fire case.

5. Cooling Water Failure Cooling Water failure will result in the loss of cooling to the condenser, which may result in failure of heat removal from the system and subsequent accumulation of vapor, which could overpressure the system. This scenario is similar to cooling failure to condenser.


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Note: Cooling water failure cases should be referred to cooling failure to condenser scenarios only if the cooling medium to the entire condenser duty is cooling water. However, if the associated PSVs discharge to atmosphere – do calculations on the worst case (i.e. whichever results in loss of higher cooling duty) and refer the other case to the worst case.

Column PSV Sizing

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