Standard Operating Procedure
Guidelines for Compressor PRA
Made by
Amol Kamble
Reviewed and Approved
Saurabh Kapse
Document No:
Page:
1 of 9
RSERA-011
Supersedes:
N/A
Previous Rev. Date:
N/A
Rev.
0
Current Revision Date:
24/03/2015
1.0 PURPOSE OF PROCEDURE
This procedure provides the guidelines for pressure relief analysis of Compressors.
2.0 SCOPE OF APPLICATION
This technical guideline is mainly intended for use of process engineers involved in pressure relief
analysis of compressors.
3.0 DEFINITIONS
Terms have been defined as and when they appear in the procedure.
4.0 RESPONSIBILITIES
The individual assigned for pressure relief analysis PRA of compressor along with the reviewer is
Responsible for implementation of procedure.
5.0 IMPLEMENTATION
This is to be implemented for all PRA projects executed by Ingenero.
6.0 PROCEDURE
6.1 Introduction: Compressor is a machine that is used to transport gases from a level of lower energy to
a level higher energy. A Compressor is distinguished from blower and fan as,
1. Fan: A machine that can develop pressure up to 1.01 psi
2. Blowers: A machine that can develop pressure up to 36 psi
3. Compressor: A machine that can develop pressure more than 37.7 psi
6.2 Type of compressors
There are basically two types of compressors
1. Dynamic type compressors (Centrifugal Compressors)
In dynamic type of compressors, velocity and pressure are transferred to the gas by the action of rotating
vanes or impellers.
2. Positive displacement compressors
In positive displacement type compressors, successive volumes of the air or gas are restricted within a
closed space and the pressure of gas is increased by decreasing volume of closed space.
Positive displacement compressors are constant volume machines that typically can develop high
discharge pressures if blocked-in.
Guidelines for Compressor PRA
RSERA SOP-011
Rev.0
Page 2 of 9
4 May 2015
---------------------------------------------------------------------------------------------------------------------------------------------Common types of positive displacement compressors include reciprocating, screw etc. The compressor
type can often be determined based on the P&ID symbol or the compressor specification sheet.
6.3 Inter-cooling
When gas is compressed, a work is done on the gas and therefore its temperature increases. This rise in
temperature has number of disadvantages as,
1. Volume of gas increases due to increase in temperature and therefore energy required to compress this
high temperature gas is larger than if compressor is isothermal.
2. Excessesive temperature of compressed gas leads to problem with lubricants, stuffing boxes and
material of construction.
3. The gas may not tolerate high temperature. It may decompose.
Therefore, to overcome above disadvantages, a intercooling is provided in-between two stages of
compressor.
6.4 Compression Ratio
The compression ratio (R) is defined as the ratio of compressor absolute discharge pressure to absolute
suction pressure. If the compression ratio is high, the temperature of the discharged gas becomes high.
Also, the heat generated due to friction during compression gets added to this. In order to remove this
heat, compressors such as reciprocating ones are cooled by jackets through which normally cold water is
circulated. Also, at compression ratios above 5.0, the gas is compressed in more than one stage.
After each stage of compression, heat of compression is removed by circulating through interstage coolers
before proceeding to next stage of compression. The intercooling not only reduces gas temperature and
volume of the gas but also reduces the horsepower required for compression.
A compression ratio of 3.5 to 4.0 per stage is considered maximum for normal process operations. A
higher compression ratio per stage means, a higher temperature rise per stage. This temperature
rise is a limiting factor for higher compression ratio. Therefore more than one stage is used for
compressing gas to very high pressures.
6.5 Surge Phenomena in compressor
The operation of a centrifugal compressor becomes unsuitable below a certain minimum capacity. This is
because of a phenomenon called as surging, which takes place below a certain minimum capacity
operation of the compressor.
In the normal operation of a Centrifugal Compressor, the discharge valve remains fully open. Suppose, if
we throttle (slightly close) the discharges valve, the system resistance or backpressure to the compressor
discharge flow increases. To overcome this resistance, the head required by the compressor must
increase.
As the throttling of discharge valves is continued, gas flow through the compressor will become less and
less. This continues up to the point of maximum head capability of the compressor. If the system
resistance still continues to increase, then, at this point a reverse flow of gas from system to compressor
will take place.
When this back flow occurs, the system resistance or backpressure gets lowered and makes the
compressor capable of delivering flow higher than when back flow or surge began. Now if the discharge
valve condition remains the same, very soon the flow will drop below the surge limit causing another back
flow. This cycle of repeated flow reversal, as the compressor alternatively tries to deliver the gas and the
system returning it is called surging. Surge may be caused by a system disturbance or insufficient suction
flow.
Guidelines for Compressor PRA
RSERA SOP-011
24 March 2015
Page 3 of 9
---------------------------------------------------------------------------------------------------------------------------------------------6.6 Surge Prevention
As a thumb rule, a centrifugal compressor surges when its capacity is reduced to around 60% of design
flow and its head exceeds 115% of Design. Surging of the compressor can be avoided by avoiding this
condition of operation.
Typically a centrifugal compressor is provided with an anti surge control to prevent surging condition of the
compressor during normal operation of the plant. The most common method is to bypass the discharge
flow back to suction through a control valve, which is called as Anti-surge valve.
6.7 Positive Displacement Compressor Analysis
The following scenarios should be done on the positive displacement compressors.
1. Closed outlets
PD compressors are constant volume machines, which can develop very high pressure. Therefore, for PD
compressors, closed outlet is always applicable scenario.
2 Relief rate based on PD compressor capacity
As per project guidelines, the relief rate will be normal operating flow rate OR rated capacity OR
mechanical design capacity of the compressor.
The relief temperature should be the discharge
temperature at the relief pressure, which should be calculated isentropically using the suction conditions,
vapor k, and the relief pressure.
Refer attached excel “PD Compressor Capacity & Disch temp
calculation.xls” for relief temperature calculations.
Caution: while calculating relief rate for blocked outlet of compressor, make sure that recycle stream
rate has been EXCLUDED from to total relief rate especially in reactor system.
3. Relief temperature calculation for closed outlet scenario
For blocked outlet scenario, relief temperature should be calculated by using following isentropic equation.
Guidelines for Compressor PRA
RSERA SOP-011
Rev.0
Page 4 of 9
4 May 2015
---------------------------------------------------------------------------------------------------------------------------------------------4. Relief rate based on the Mechanical Design Capacity of the compressor
This is alternative method to calculate the required relief rate for PD compressor. This method use physical
dimensions for the compressor, Cylinder dimensions, stroke length, head end clearance, Rod diameter,
and Engine speed.
5. Cooling failure to intercooler: This scenario should be done if there is interstage cooling provided. It is
particularly applicable in multistage PD and centrifugal compressors. Note that, this scenario should be
analyzed on the discharge knock out drum and not on the compressor.
Note: If relief valve location is in-between compressor discharge and upstream of intermediate cooler, this
scenario will be same as closed outlet on the compressor since for both the scenarios, rate and relief
temperature will be same.
6. Check valve failure: failure of discharge check valve scenario should be identified on the compressor.
Refer check valve SOP for further information.
Note: A compressor should be considered a single check valve for upstream system, as compressor itself
acts as a check valve for upstream vessel.
7. Failure Open of Kickback Valve under Running Condition
1. The scenario associated with the failure open of the kickback valve while the compressor is still running,
resulting in increased circulation flow through the system, should be evaluated. Note that while it is not a
necessary consequence that the flow into the system reduces or the flow out of the system increases (due
to decreased/increased pressure differentials, respectively, it is nevertheless possible.
Guidelines for Compressor PRA
RSERA SOP-011
24 March 2015
Page 5 of 9
---------------------------------------------------------------------------------------------------------------------------------------------2. Calculate the settle out P based on the available volumes on the high pressure and the low pressure
side. If the calculated settle out P < the relief valve set P on the compressor suction side, then the case is
Not applicable.
3. If the settle out P > the relief valve set P, overpressure is expected. Relief rate is calculated as follows:
Conservatively, the pressure upstream of the compressor can be assumed to be at relief pressure
Calculate the new discharge pressure based on the new suction pressure
(If there is a discharge PRV and the newly calculated pressure exceeds the set pressure of the PRV, stop
at the discharge relief pressure).
Calculate the flow across the control valve instantaneously from discharge pressure to relief pressure and
use this rate as the required relief rate.
Single check valve failure (when compressor shuts down)
Not applicable when downstream system (HP system) volume content is not sufficient to overpressure the
upstream system (LP system) available volume, then overpressure is not expected.
This is only valid for a HP system with limited mass inventory or with feed sources that do not exceed
the upstream system (LP system) MAWP. Calculation of the relative volumes and settle-out pressure
must be documented in the relief systems design basis.
Guidelines for Compressor PRA
RSERA SOP-011
Rev.0
Page 6 of 9
4 May 2015
---------------------------------------------------------------------------------------------------------------------------------------------Centrifugal Compressor Analysis
Dynamic Compressors mechanically impart a velocity to the air through the use of impellers rotating at
high speed in an enclosed housing. The dynamic compressor volumetric flow will vary inversely with the
differential pressure across the Compressor.
The centrifugal compressor has typical following scenarios.
1. Closed outlets (Normal Suction/Normal Discharge)
2. Closed outlets (Max Suction/Max Discharge)
3. Check valve failure
i. A typical centrifugal compressor curve
The head values for centrifugal compressor calculation will be based on the performance curve. A typical
curve is shown as follows.
Guidelines for Compressor PRA
RSERA SOP-011
24 March 2015
Page 7 of 9
---------------------------------------------------------------------------------------------------------------------------------------------ii. Centrifugal compressor discharge pressure calculations
Normal Suction/Maximum Head
1. Enter the normal suction pressure from PFD/datasheet/P&ID, in given order of preference.
2. Enter the normal suction temperature from PFD/datasheet/P&ID, in given order of preference.
3. Enter mass % of vapor, molecular weight, vapor k, Vapor Z and polytropic efficiency.
4. Enter normal and maximum differential head developed by compressor. Maximum differential head
should be obtained from compressor curve.
Maximum Suction/Maximum Head
5. Enter the maximum suction pressure from PFD/datasheet/P&ID, in given order of preference.
6. Enter the maximum suction temperature from PFD/datasheet/P&ID, in given order of preference.
7. Enter mass % of vapor, molecular weight, vapor k, Vapor Z and polytropic efficiency.
8. Enter normal and maximum differential head developed by compressor. Maximum differential head
should be obtained from compressor curve.
iii. Check valve failure (Upstream vessel): failure of discharge check valve scenario should be identified
on the compressor. Refer check valve SOP for further information.
Guidelines for Compressor PRA
RSERA SOP-011
Rev.0
Page 8 of 9
4 May 2015
---------------------------------------------------------------------------------------------------------------------------------------------iv. Failure of automatic control valve (Spill back control valve):
The scenario associated with the failure open of the kickback valve while the compressor is still running,
resulting in increased circulation flow through the system, should be evaluated. Note that while it is not a
necessary consequence that the flow into the system reduces or the flow out of the system increases (due
to decreased/increased pressure differentials, respectively), it is nevertheless possible.
v. Compressor Settle-out Pressure
1. A compressor compresses gas from one system at P 1 & T 1 to another system at P 2 & T 2 and stops
during the maximum pressure drop case, a differential pressure is developed. After a compressor
shutdown, the gas is trapped between the upstream and downstream discharge check valve and the
pressure is equalized out. This equalized pressure throughout compressor loops is called settle-out
pressure. The maximum settle-out pressure is calculated from coincident high-trip pressures on both
suction and discharge sides of the compressor.
When the centrifugal compressor is tripped, the recycle valve goes to the open position while the
shut down valves labeled 1, 2 and 3 in the figure below are tripped close. This results in the
distribution of the trapped gas in the various blocked section, which will settle out at an intermediate
pressure between the suction and discharge pressures. This phenomenon is referred to as “settle out”.
For compressor stage K1, the settle-out pressure is calculated for the volume between shut down valves 1
and 3. For compressor stage K2, the settle-out pressure is calculated for the volume between the
shutdown valves 3 and 2.
The above is true if the two stages of the compression K1 and K2 are in separate casing. In case both
stages of the compressor are located in a single casing, there will be leakage between the two stages on
the compressor via the labyrinth seals separating the 2 stages in the casing. This will result in the 1st stage
settle out pressure to be much higher than that calculated due to independent stage settle out due to the
leakage of the higher pressure gas in compressor K2 discharge.
Guidelines for Compressor PRA
RSERA SOP-011
24 March 2015
Page 9 of 9
---------------------------------------------------------------------------------------------------------------------------------------------vi. Settle-out Pressure Calculation
To calculate settle out the pressure API 521 Appendix B recommends the settle out is calculated for the
maximum pressure drop case. For this, the suction side is considered to be at the normal operating
pressure and the discharge side to be at the maximum pressure which can be considered as the set point
of the high pressure trip at the compressor discharge.
For the calculation of the settle-out it is safe to consider the suction vessel at the maximum level and the
discharge vessel at the minimum level.
vii. Cooling failure to intercooler: This scenario should be done if there is interstage cooling provided. It
is particularly applicable in multistage PD and centrifugal compressors. Note that, this scenario should be
analyzed on the discharge knock out drum and not on the compressor.
7. REFERENCE DOCUMENTS
a) API Standard-521
8. RECORDS
Application of this procedure shall complete a technically correct Pressure Relief Analysis (PRA) of
compressors.
9. ATTACHMENTS
1. PD Compressor Capacity & Disch temp calculation.xls