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.

 

 

PILOT OPERATED PRESSURE RELIEF DEVICES

                Pilot operated pressure safety valves (POPSV) consists of main valve which normally encloses a floating unbalanced piston assembly and an external pilot. 

                        Figure: Parts of POPSV

 Operating mechanism of Pilot operated PSVs is described in following steps

Step 1: Piston is designed to have a larger area on the top than the bottom. Up to the set pressure, the top and bottom areas are exposed to the same inlet operating pressure. Because of the larger area on the top of the piston, the net force holds the piston tightly against the main valve nozzle.

As operating force increases ---- ->Net seating force increases ------>which makes valve tighter 

 The net seating force of pilot operated PSV increases with increase in operating pressure unlike the conventional PSV as shown in the below figures. 

Step 2: At set pressure, the pilot vents the pressure from the top of the piston, the resulting net force is now upward causing the piston to lift, and process flow is established through the main valve.

Step 3: After over pressure incident, the pilot will close the vent from the top of the piston, thereby re establishing the pressure and the net force will cause the piston to re seat.

The lift of the main valve piston or diaphragm is not affected by built up back pressure.

 

                 A back flow preventer (NRV) is required when the possibility exists of developing a pressure on the discharge side of the PSV that exceeds the inlet pressure of PSV.

The pilot that operates the main valve can be either a pop action or modulating action pilot.

Pop action pilot -------> Causes the main valve to lift fully at set pressure without over pressure. (Similar to gas operated Pop action conventional PSV)

Modulating pilot -------> opens the main valve only enough to satisfy the required relieving capacity. (Similar to liquid operated pressure relieving device)

 

 Also the pilots can be classified as Flowing and Non flowing

Flowing pilot allows process fluid to continuously flow through the pilot when the main valve is open and where the non flowing type does not.

Applications

·         POPSVs are used when operating pressure is above 90% of set pressure.

·         Used during high back pressures (> 50%) as valve lift is not affected by back pressure.

·         Used in critical applications where passing of PSV is not allowed as closing force of valve increases with increase in operating pressure.

 

CONVENTIONAL PRESSURE RELIEF DEVICES

Operation of conventional PSVs is based on the force balance.

Spring load (pre set) = Force exerted on the closed disc by the inlet fluid when the system pressure is at set pressure.

 

 

The above figure shows the force diagram of PSV during normal condition.

The operating mechanism of PSV can be classified into Gas / Vapour service & Liquid service.

Operating mechanism of PSVs with Gas / Vapour service is described in the following steps.

Step 1: As the system pressure approaches the set pressure of the valve, the seating force between the disc and nozzle approaches to zero.

 When the vessel pressure closely approaches to the set pressure, fluid will audibly move past the seating surfaces into the huddling chamber.

Step 2: As a result of restriction of flow between the disc holder and adjusting ring, pressure builds up in huddling chamber as shown in the below figure. It is the operation phase during initial opening.

 

 Step 3: Since pressure now acts over a larger area, an additional force is available to overcome the spring force which causes the PSV to open. This is the operation phase of PSV fully open and is shown in the below figure.

 

 By adjusting the adjusting ring, the opening in the annular orifice can be altered, thereby controlling the pressure build up in huddling chamber.

Step 4: PSV closes when the inlet pressure has dropped sufficiently below the set pressure to allow the spring force to overcome process fluid forces.

Operating mechanism of PRVs with Liquid service is described in the following steps.

Liquid service PRVs do not pop in the same manner as vapour service PSVs, since expansive forces produced by the vapour are not present in the liquid flow. Liquid service PRVs depends on reactive forces to achieve the lift.

Step 1: At initial opening, the escaping liquid forms a very thin layer, expanding radially between the seating surfaces as shown in the below figure.

 

 Step 2: Liquid strikes the reaction surface of the disc holder and is deflected downward creating a reactive force to move the disc holder upwards. These forces typically build very slowly during first 2% to 4% over pressure.

Step 3: Increase in flow leads to increase in velocity which further leads to increase in reactive forces. Increase in reactive forces finally causes the PRV to lift.

Typically PRV will suddenly surge to 50% to 100% lift at 2% to 6% over pressure.

Liquid PRV during operation is shown in following figure.

 

 Step 4: After relieving, the decrease in over pressure leads to decrease in reactive forces which further causes the PRV to close.

During operation of a conventional PSV, the disk lift vs pressure acting on the pressure relieving device is shown in the below figure.

 

Balanced Pressure relief valves

There are mainly two purposes for incorporating bellows in pressure relieving devices.

1.       Minimising the back pressure

Balanced PRVs are the pressure relieving devices which incorporates bellows for balancing the valve disc to minimise the effects of back pressure.

A bellow is attached to the disc holder with a pressure area, approximately equal to the seating area of the disc. This minimises the effect of back pressure acting on it.

2.       Acting as a seal to top parts of PSV

As bellow is attached to the disc holder, this isolate the top parts of the PSV like guide, spring, bonnet within the PSV from the relieving fluid.

The bonnet of a balanced PSV must be vented to atmosphere for the bellows to perform properly.

If any leak is observed from bonnet vent then it is an indication that a bellow is failed as it is unable to isolate the top parts of the PSV.

Incorporating a bellow is very important if there is concern that the fluid will cause corrosive damage to other parts. Where as in conventional PSVs without bellows, during operation, the relieving fluid flows between disc holder and guide, thereby building up the bonnet pressure. This adds variable force to the spring force, which inhibits the PSV lift.

Also this is the reason that a bonnet vent shall be always closed for conventional PSVs without bellows to avoid fluid spillage to outside atmosphere.

The bonnet pressure is reduced by installing educator tube on the hole fabricated on the guide as shown in the following figure.

Balanced PSVs are typically installed where total back pressure (Super imposed + built up) does not exceed approximately 50% of the set pressure.

The force diagram of bellow PSVs can be shown below.

From the above diagram, the force balance equation can be derived as follows.

P1 * A1 = Fs

Where P1 = Inlet pressure acting on PSV disc.

                A1 = Area of the disc.

                Fs = spring force.

If bellows are absent, then 

P1 * A1 = Fs + Pb * A1

Where Pb = Super imposed back pressure.

 Figure reference of force diagram (bellows absent case) can be taken from disc lift vs pressure

 

PRESSURE RELIEF DEVICE – TERMINOLOGY

                Pressure relief devices are the devices actuated by inlet static pressure and designed to open during emergency or abnormal conditions to prevent a rise of internal fluid pressure in excess of a specified design value.

Figure - PSV cross sectional view

These devices are designed to open and relieve excess pressure and to re close and prevent the further flow of fluid after normal conditions have been restored.

Type of valve \ Property	Loading type	Valve opening	Fluid type
Relief valve	Spring loaded	Proportional to pressure rise over opening pressure	Incompressible fluids (liquid service)
Safety valve	Spring loaded	Rapid or Pop action	Compressible fluids (Gas service)
Safety relief valve	Spring loaded	Used as safety valve or relief valve depending on the application	Multi service applications (foam like) [Two phase mixture at valve inlet is 50% vapour or less]

 

Conventional pressure relief device

Ø  Spring loaded device

Ø  Operational characteristics are directly affected by changes in the back pressure.

Figure - Conventional and Bellow PSV

 

Balanced pressure relief valve

Ø  Spring loaded device

Ø  Incorporates bellows or other means for minimizing the effect of back pressure on the operating characteristics of the valve.

Pilot operated relief valve

Ø  Major relieving device or generally main valve is combined with and controlled by a self actuated auxiliary pressure relief valve called pilot.

Figure - Pilot PSV

Huddling chamber

An annular chamber located downstream of the seat of a pressure relieving device for the purpose of assisting the valve to lift.

 

Pressure

In this contest, Pressure can be mainly divided into two categories.

1.       System pressures &

2.       Device pressures

System pressures

·         Maximum operating pressure is the maximum pressure expected during normal system operation.

·         Maximum Allowable Working Pressure (MAWP) is the maximum gauge pressure permissible at the top of a completed vessel in its normal operating position at the designated coincident temperature specified for that pressure.

Or

It measures the greatest amount of pressure that the weakest part of the vessel can handle at specific operating temperatures.

·         Design pressure of the vessel along with the design temperature is used to determine the minimum thickness or physical characteristic of each vessel component as determined by vessel design rules.

It is selected by the user to provide a suitable margin above the most severe pressure expected during normal operation at coincident temperature.

Design pressure less than or equal to MAWP

MAWP is the highest pressure it could be exposed to.

Design pressure is the highest pressure it should be exposed to, during normal operating conditions.

·         Accumulation is the pressure increase over MAWP of the vessel during discharge through pressure relieving device.

·         Over pressure is the pressure increase over the set pressure of the relieving device allowed to achieve the rated flow.

 

Figure - Pressure level relationships for pressure relief valves

 

Design pressures

·         Set pressure is the inlet gauge pressure at which the pressure relief device is set to open under service conditions.

·         Cold Differential Test Pressure (CDTP) or Cold Differential Set Pressure (CDSP) is the pressure at which a pressure relief valve is adjusted to open on the test bench.

It includes correction for the service conditions of back pressure or temperature or both.

·         Back pressure is the pressure that exists at the outlet of a pressure relief device as a result of the pressure in the discharge system.

Back pressure = Super imposed back pressure + Built up back pressure.

·         Built up back pressure is the increase in pressure at the outlet of pressure relief device that develops as a result of flow after pressure relief valve opens.

·         Super imposed back pressure is the static pressure that exists at the outlet of a pressure relief device at the time of device required to operate.

For pilot and balanced bellows type relief valves, effect of back pressure on valve opening characteristics is very low and hence CDTP is same as the set pressure.

 

For example, if PSV installed in a process system is designed to relieve at 10 bar and the PSV discharge is connected to the flare header having 1 bar, then

 

Set pressure = 10 bar

CDTP = 9 bar

Back pressure = 1 bar

 

·         Blow down is the difference between set pressure and the closing pressure.

·         Simmer is audible or visible escape of compressible fluid between seat and disc which may occur at inlet static pressure below the set pressure prior to opening.

·         Operating ratio = Maximum system operating pressure / Set pressure.

For any industry PSV is considered as one layer of safety protection to the plant. So it is the safety critical equipment of the industry for which no chance has to be taken in the maintenance perspective.