Oversize PRV and chattering 1

[RZD13]

Hello everyone.
I would like to ask if the PRVs installed at my company's gas metering station are oversized and will they cause chattering should they lift.
Below is the information:

PRV - pilot operated type
Fluid - Natural Gas
Molecular weight/SG - 16.38
Operating pressure - 17 to 19 barg
Operating temperature - 20 to 38 °C
Relieving temperature - 38°C
Design temperature - 50°C
Design pressure - 20 barg
Set pressure - 21 barg
Allowable overpressure - 20%
Compressibility factor - 1
Coefficient of discharge - 0.97
Ratio of specific heats - 1
Back pressure - 0 barg
Required flowrate - 182.3 x 10³ SCMH
Selected orifice area - 5.064 cm²
Required capacity - 0.236 Sm³/s
Max Capacity - 6410 kg/h

If you need any more infos, I will provide them.
Thank you.

 

[pierreick]

Hi,
Consider the link attached to support your query.

https://www.linkedin.com/pulse/chattering-safety-valves-stefan-lange

Good luck
Pierre

 

[The Obturator]

You can expect to get better commentary in reply if you could also advise on:-
[ul]
[li]Size and type of pilot operated Pressure-relief Valve (pop action or modulating design? - manufacturer? model no?)[/li]
[li]Installation details - type of vessel[/li]
[li]Has any inlet pressure drop been ascertained?[/li]
[li]Is there any known incident of the existing valve chattering?[/li]
[li]What is the reason for the question after the valve has been installed?[/li]
[/ul]

Can you also clarify the required capacity the valve is required to pass as a minimum. You have stated required flowrate, required capacity and maximum capacity which are all different.

Chattering can be caused by many factors either by a single issue or a number of them.


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[RVAmeche]

A "basic" review of if it a PSV is oversized is comparing the determining required flow/area (the worst case sizing scenario, which hasn't been provided) vs the installed valve's flow/area (which you've provided). The other half is reviewing the 3% rule - at the valves rated capacity, what is your inlet pressure drop? If its greater than 3% that indicates generally indicates chattering could occur.

Note chattering is a complicated issue and the 3% rule alone doesn't guarantee chattering will/won't occur, but its still the industry norm at the moment.

 

[georgeverghese]

Is this some kind of prank ? Nothing makes sense here.
a) There is no such thing as Cp/Cv = 1.0 for natural gas at 20barg
b) Required capacity doesnt match required flowrate
c) Why would you set the PSV at 21barg when design pressure = 20barg

 

[Snickster]

Design pressure 20 barg and relieving pressure 21 barg. lol.

 

[Snickster]

Required capacity = .236 Sm3/s = 849.6 scm3h

Required flowrate = 182300 scm3h

LOL

 

[Snickster]

Coeficient of specific heat for natural gas = 1.27 lol. busted!

 

[georgeverghese]

Probably has some thing to do with today being April Fools Day

 

[Snickster]

20% overpressure is for fire case only. lol.

 

[pierreick]

Hi,
For those interested with the subject.
Pierre

 

[shvet]

I would like to ask ... will they cause chattering should they lift.

You confuse chattering and cycling. Note that many companies ignore cycling as that is not so dangerous as chattering is.

2.4.2.2.4 Oversizing
A safety valve size is selected to handle the estimated peak venting load with some capacity to spare. Thus, the valve is actually oversized, particularly for events of lower severity than the design case. The result is open-and-close cycling for pop-action devices. This relatively slow cycling is not to be confused with destructive chatter, which results from the piping being undersized with respect to the valve capacity. Nevertheless, gross oversizing of the system is to be avoided. Good maintenance practice dictates that valves be checked for seat tightness and set point if a venting incident causes prolonged valve cycling. Use of multiple valves with staged sizes and set pressures will reduce cycling problems.

For info

7.1.1 PRV Cycling
Cycling is the relatively low frequency (e.g. a few cycles per second to a few seconds per cycle) opening and closing of a relief valve. This most often occurs when the relief requirement is small when compared to the capacity of the valve. In this case, when the PRV opens, the valve may flow more than what the system can provide, causing the pressure to drop to the PRV’s reseating pressure. Once the PRV is closed, the system pressure rebuilds to the PRV set pressure and the cycle repeats. Cycling frequency is a function of the upstream system’s ability to keep the valve open and is much lower than the natural frequency of the valve. In general, cycling does not cause detrimental valve damage. However the valve’s ability to reseat tightly may be affected and it may cause some wear over time.
When capacity variations are frequently encountered in normal operation, one alternative is the use of multiple, smaller PRVs with staggered settings. With this arrangement, the PRV with the lowest setting will be capable of handling minor upsets, and additional PRVs will open as the capacity requirement increases. Refer to API 520 Part I, to determine set pressure of the PRVs based on maximum allowable pressure accumulation for multiple valve installations. An alternative to the use of multiple PRVs with staggered settings is the use of a modulating pilot-operated relief valve.

1265 Required Relief Devices
...
Required Device Size
Size each relief device so that it will have adequate flow capacity for all causes of overpressure applicable to the equipment item or items it is intended to protect. Often the causes of overpressure that requires the largest relief valve orifice or rupture disk diameter can be determined by direct examination of the required flow rates. However, when contingencies differ with respect to the relief fluid’s composition, phase, or other property, the largest required flow rate may not correspond to the largest required relief device size. Therefore, after sizing the relief device for what appears to be the contingency requiring the largest device, the device should be evaluated and documented for all causes of overpressure identified, to ensure adequate capacity.

10. Pressure relief devices
10.1. General
a. Equipment, vessels, or process systems protected with more than one relief device for the same overpressure contingency should have staggered set pressures for these relief devices as follows:
1. A smaller relief valve with a lower set pressure for smaller and more frequent upset loads should be considered in addition to the larger valve for the less frequent, larger relief loads. Typically, a relief valve may be considered oversized if inlet flow is less than approximately 25% of capacity.

7.5 PRESSURE RELIEF VALVE CHATTERING
Chattering is the rapidly alternating opening and closing of a PR valve. This vibration may result in misalignment and leakage when the valve returns to its normal closed position. If chattering continues for a sufficient period, chattering may result in mechanical failure of valve internals or associated piping fittings. In addition, the vibration may loosen bolts and result in flange leaks around the PR valve. Chattering may occur in both liquid and vapor service PR valves. The principal identified causes of PR valve chattering are oversized valve, excessive inlet pressure drop, excessive built-up back pressure incorrect blow-down ring setting, and liquid surge. In addition, a further mechanism of chattering may be introduced in some liquid service PR valve installations if the response characteristics of a control valve in the same system are such that hunting between the two occurs. Generally, this can be eliminated by adjustment of instrument settings or by installation of two valves with staggered set points. The lower set valve should be sized to handle about 25% of the required
capacity.
7.5.1 Oversized Valve
“Pop" action PR valves in vapor service open at the set point by the action of static process pressure on the valve disc, and move to full open position at only a small overpressure. Typically, a flow through the valve equal to at least 25% of its rated capacity is necessary to keep the disc in the open position. At lower rates, the kinetic energy of the vapor flow is insufficient to keep the valve open against the action of the spring and it returns to the closed position, only to reopen immediately since the static pressure within the system still exceeds the set pressure. Chattering results from continuous cycling in this manner. It can occur when a "pop" type PR valve is too large for the quantity of flow being discharged. In most cases, the use of multiple PR valves with staggered set points may be appropriate to eliminate this problem, as described later in this section. Liquid service PR valves are characterized by progressively increased lift with increasing inlet pressure, rather than the “pop" action of vapor service valves. Liquid service valves are, therefore, less likely to chatter at low relieving rates, and they will modulate down to about 10% of rated capacity before chatter becomes a problem. However, pumps with very flat capacity curves can (and have often) resulted in PR valve chattering when the PR valve is oversized.
...
7.6.2 Preventing Chattering
In PR valve sizing it is always necessary to select the next larger commercially available orifice above the calculated size. Furthermore, a PR valve may lift as a result of various contingencies, any one of which requires a lower relieving rate than the design contingency. Both these factors affect the likelihood of a vapor PR valve chattering in service, since chattering (as described previously) is more likely to occur when the quantity of fluid being discharged is less than about 25% of its rated capacity. Where different contingencies of equal probability require substantially different capacities, it is always best to use two or more PR valves with staggered settings. For example, if one contingency required a capacity of 25,000 lb/h (3 kg/s) and another 100,000 lb/h (12 kg/s), two PR valves would be used, with one of 25,000 lb/h (3 kg/s) and the other 75,000 lb/h (9 kg/s) minimum capacity. The lower capacity valve in this case would be at the lower staggered set pressure. When a fire contingency is the largest contingency and the next largest contingency is less than 25% of the fire relieving rate, multiple PR valves with staggered settings should always be used. However, when the fire contingency is the smallest load, it is generally ignored. This is because fire is a rare occurrence and chattering under fire conditions is not a significant concern.