PSV CALIBRATION / INSPECTION & TESTING & FREQUENCY

Calibration is the process of comparing an unknown value with a known value. The process might be involving in determining, checking or rectifying.

PSV calibration is the process of setting the PSV pop pressure to its design value of pop setting.

PSV calibration is done by 3 methods.

1.       Offline calibration – PSV calibration on test bench

2.       Online calibration (In situ) – PSV calibration in installed condition with Nitrogen

3.       Online calibration (In situ) – PSV calibration in installed condition by Trevi method

The procedure followed for each method is explained below.

Procedure for PSV calibration on test bench

1.       Ensure upstream isolation valve of PSV is not passing before removing PSV from its installed position to avoid gas / service fluid leak to atmosphere.

2.       Ensure process isolation of PSV i.e vent the gas / liquid entrapped in PSV inlet / outlet lines. Close the inlet and outlet isolation valves of PSV.

3.       Remove PSV from installed position and shift the PSV to test bench. Ensure mechanical isolation by installing blinds to inlet and outlet flanges of the PSV after removal.

4.       Fix the PSV on the test bench. Secure it using the test clamps provided for the test bench. Ensure that PSV outlet is facing towards opposite direction of the operator to avoid PSV venting on to the working personnel during pop test.

 Figure: PSV Test bench

5.       Now PSV set pressure is checked by increasing the pressure at the inlet of the PSV with Nitrogen by using Nitrogen cylinder connected to the test bench. Use water for liquid service.

6.       If PSV pops at CDTP (Cold Differential Test Pressure) then no further action to be taken in adjusting setting of PSV. As calibration activity is on test bench and no back pressure involved during test, CDTP shall be used.

7.       If PSV pops before / after the CDTP then adjust spring lock nut by tightening / loosening accordingly.

8.       After PSV calibration, Seat tightness test and leakage rate test shall be done.

a.       Seat tightness test – Hold the inlet pressure of PSV at 90% of the set pressure (if set pressure > 3.4 bar) & at 5 psi (if set pressure < 3.4 bar) and check for any drop in holding pressure. If no drop observed it is ok and leakage rate to be checked if any drop observed.

b.      Leakage rate test – Hold the rubber plug or any leak proof plug against the outlet of PSV and observe for any bubbles in the bubble counter cup.

9.       Seat tightness test and leakage rate test shall meet the requirements of API 527.

10.   If any leak observed beyond the limit then PSV servicing (disc lapping) to be done.

11.   Bellow test shall be carried out for PSV, if bellows were installed. Steps for bellow test are as follows.

a.       Blind the outlet flange of the PSV.

b.      Maintain the inlet pressure of PSV to 1 bar on the test bench.

c.       If bellow is leaking then bubbles will be observed from the bonnet of PSV.

12.   If PSV fails in bellow test, then bellow has to be replaced with new one.

13.   Now, after completion of all the tests, unclamp the PSV from the test bench, shift the PSV to its installed position and box up shall be done.

14.   Ensure process normalisation after installing the PSV in its position.

Procedure for Online calibration (In situ) – PSV calibration in installed condition with Nitrogen

If PSV dropping from installed position is not possible for calibrating on the test bench, then this method is followed.

1.       Ensure upstream isolation valve of PSV is not passing before removing PSV. If upstream isolation of PSV is found passing, then the Nitrogen given for calibration might be going into the system or service fluid from the system might be coming into the test manifold or test equipment.

2.       All the isolation valves are to be maintained as per the mark up P&ID which is to be done before executing the job. Sample P&ID is shown in the below figure.

3.       Upstream isolation valve (U1) of PSV shall be closed to avoid mix up of service fluid and Nitrogen used for calibration and to record the correct pop up pressure.

4.       Downstream isolation valve (D1) of the PSV shall be in open to let out the Nitrogen to flare or safe venting location after PSV pop up.

5.       If stand by PSV available for system, then ensure stand by PSV to be kept in line as shown in the below figure.

6.       Connect Nitrogen hose from nitrogen cylinder to ¾” vent (V1) in upstream line of PSV.

7.       Now PSV set pressure is checked by increasing the pressure at the inlet of the PSV with Nitrogen by using Nitrogen cylinder connected to the test equipment / manifold. If the PSV set pressure is high, then booster compressor is also used along with test manifold.

8.       Now PSV set pressure is checked by increasing the pressure at the inlet of the PSV with Nitrogen by using Nitrogen cylinder connected to the test manifold.

9.       If PSV pops at set Pressure then no further action to be taken in adjusting setting of PSV.

10.   If PSV pops before / after the set pressure then adjust spring lock nut by tightening / loosening accordingly.

11.   Seat tightness test, leakage rate test and bellow test can’t be performed in this method which is drawback of this method.

12.   Remove Nitrogen hose from the system and normalise the isolation valves of the PSV and PSV can be taken into service.

Procedure for Online calibration (In situ) – PSV calibration in installed condition by Trevi method

If PSV dropping from installed position is not possible for calibrating on the test bench, then this method is followed. In addition to this method can also be used when upstream isolation valve is in either open condition or closed condition.

For PSVs with upstream isolation valves are passing, the upstream isolation valve can be kept in open condition and calibration can be done.

This method is also complied with API RP 576.

The equipment used for trevi method is shown below.

In this method instead of giving pressure at inlet of the PSV, the same amount of force is applied in upward direction with the help of displacement transducer and hydraulic / electric load cell installed on the spindle of PSV.

Procedural steps are as follows.

1.       Remove PSV cap and install hydraulic / electric operated load cell on the valve spindle which further connects to trevi test equipment and software in laptop. Ensure proper tightness of load cell to valve spindle.

2.       All the inputs like operating pressure or system pressure, set pressure, re seat pressure and area of the disc shall be entered into the software before performing the test.

3.       Line diagram of this test method can be shown as below.

 

4.       Apply force to PSV spindle through load cell. If hydraulic system is used ensure no storage volume shall be present during connecting and disconnecting.

5.       Using software monitor the force applied on the PSV spindle. Ensure downstream of the PSV in open condition to let out the service fluid to safe vent location.

6.       After completion of the test, the required parameters like set pressure, re seat pressure are obtained which were calculated from the graph (Force vs time).

7.       If PSV pops at set pressure then no further action to be taken in adjusting setting of PSV.

8.       If PSV pops before / after the set pressure then adjust spring lock nut by tightening / loosening accordingly and again set pressure shall be verified by performing the test again.

9.       Seat tightness test, leakage rate test and bellow test can’t be performed in this method which is drawback of this method.

10.   Disconnect load cell and hoses from the position after completion of PSV calibration. Ensure storage volume shall not be present, if it is a hydraulic hose.

11.   Normalise the isolation valves of the PSV and PSV can be taken into service.

Example of PSV calibration by Trevi test method:

Details of the PSV are as follows

Set pressure = 7.2 bar

System pressure = 3 bar

Area of the disc = 167.7 cm2

                As already system is having 3 bar pressure, the effective net pressure = 7.2 – 3 =4.2 bar

                                                                                                                                                =4.2 * 1.01 kg/cm2

                                                                                                                                                =4.242 kg/cm2

Net force to be pulled = P * A

                                = 4.242 * 167.7

                                =711.3834 kg

                                =711.3834 * 0.0098 KN

                                =6.97155 KN

                So, if Net force is 6.9 KN, then PSV pops At 7.2 bar, otherwise PSV has to be adjusted accordingly.

 

Limits of PSV set pressures

1.       Set pressure can be set with a tolerance of + (or) - 3%. [If set pressure >  4.83 bar] {API RP 576}

2.       Set pressure can be set with a tolerance of 0.138 bar. [If set pressure < or =  4.83 bar] {API RP 576}

3.       Change in set pressure shall be done within a range of + (or) – 5%. If manufacturer allows more than 5%, then this can be done. {ASME BPV Sec VIII, Div 1}

Inspection & testing / Calibration frequency of PSVs

As per OISD 132, the compliance of inspection frequency is shown in below table.

Safety relief device application	Compliance
Steam boiler service	Government boiler regulation
LPG / Gas storage	SMPV rules
Hydrocarbon service (Process area)	1st inspection - With in 2 years of commissioning Next inspection - Based on operating history / every turnaround, but in no case later than 5 years
Hydrocarbon service (Offsite area)	1st inspection - With in 3 years of commissioning Next inspection - Based on operating history / every turnaround, but in no case later than 5 years
Utility service	Every turnaround but not more than 10 years
In situ inspection of PRV / VRV / PVRV	Once in 6 months
Pilot operated valves	Once in a year for valves installed in fouling services / Once in two years for valves installed in clean services
Safety relief device application (E&P)	Compliance
Offshore applications	All PSVs installed in offshore (E&P) installations shall be tested for operation at a frequency atleast once in 12 months. These valves shall be either bench tested or equipped to permit testing with an external pressure source. Inspection and subsequent testing of PSVs shall be carried out atleast once in every 5 years or as per the valve manufacturer which ever is earlier.
Onshore applications	All PSVs installed in onshore (E&P) installations shall be tested inline with the requirements of oil mine regulations 1984. Inspection and subsequent testing of PSVs shall be carried out atleast once in every 5 years or as per the valve manufacturer which ever is earlier.