PSV Chattering for ASME and API Safety Valves 4

[Morteza Sadeghinia]

Hi,

Would you please let me know what parameter in PSV sizing (both API and ASME SEC VIII) output shows that a valve could suffer from chattering? What's the reference and acceptable criteria? Is it somehow related to capacity exceed?
Thanks

 

[The Obturator]

Whether you size using the API (effective area) or ASME (actual area) method, does not matter. API sizing is preliminary sizing done in order to select a size against various vendors and ASME sizing is done with the selected vendor and design of PRV. Actual sizing should always be used if you know which valve you are to use.

There is no written advice on when a PRV will chatter. Most people will tell you that you require a calculated area of 30 to 40 % or less (of the selected area) for chattering to occur. That is a very broad statement and there are many other reasons why chatter can and will occur.

The subject has been covered extensively in the past in this forum and you find many viable comments by just entering 'chattering valve' or 'valve chatter' etc., in the search box above.

I'll list out some common issues leading to PRV chatter.
Note that chatter is the very rapid and closing of the disc making contact with the fixed nozzle (seat). There is also another phenomenon called Flutter, where the disc is moving up and down but without touching the seat. Flutter can also be quite damaging and occurs where the PRV cannot cleanly close (blowdown).

Chatter of a PRV can occur with the following issues, or possibly some of them together;-

1. Oversized PRV - Some say you require at least 30 to 40 % calculated area, That is no guarantee and it is worser for non compressible fluids ie., liquids. In 2017 API-526 recognised the use of restricted lift spring operated PRV. This is also now in ASME VIII. Manufacturers have had to recertify flow capacities for existing designs. Restricted lift alone solves the age old oversized PRV problem, but some rules need to followed.

2. Inlet pressure losses of 3 % or more - This basically starves the PRV of capacity and chattering occurs. Can be from the excess length of inlet pipe (in which case increase pipe diameter), geometry of bends etc.

3. Inlet (and outlet) pipe (bore) size less than the PRV inlet and outlet nominal size anywhere in piping before and after PRV. Nor should there be any obvious obstruction in the pipe like a meter or gate valve (unless rules are followed)

4. Excess back pressure (against wrongly sized and/or selected PRV).

5. Incorrect nozzle ring setting (often <wrongly> called the blowdown ring) - often wrongly set after maintenance.

6. Varying flow rate - possibly from situations where the PRV has been selected on 'worst case' which is hardly seen and other 'lower' flow cases are going into overpressure.

7. PRV design - Many PRV's are installed on liquid application with standard gas trim. Since 1985 ASME has recognised liquid systems and required manufactrers to develop liquid PRV's to open fully at 10 % overpressure. These days, some manufacturers offer multi or universal trims which have also been flow tested.

8. Process laterals installed in the inlet piping leading to the PRV can influence PRV opening.

9. Wrongly sized (or misunderstood) multi phase flows (2 phase flow sizing method).

10. Bursting disc (if fitted) not considered in PRV application.

Etc., etc., there can be more issues, and as you can see, many of the issues contributing to PRV chatter are the installation of the PRV.



Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[The Obturator]

I will add that my earlier comments, in the main referred to spring operated pressure relief valves.

Pilot operated pressure relief valves do not generally experience chatter due to their design being either;-

Snap (or pop) acting design for compressible fluids - where the main valve is fully open at set pressure.
or
Modulating design - where the main valve will open in proportion to the rise of overpressure.

Dependant on application, POPRV's have solutions where chattering occurs with spring designs eg., inlet pressure loss can be resolved by having a remote sensing line directly from the pilot to the protected vessel. Also, POPRV's are not affected by back pressure in the same way as spring designs.


Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[Morteza Sadeghinia]

Thank you so much.
What you've mentioned is basically cases which might lead too PSV chattering.
But just by looking at PSV calculation, is that possible to know whether chatter will occur? Someone told me once that capacity exceed more than 200~300% will cause chattering. Is that a correct statement?
Frankly I'm trying to somehow find a relationship between PSV calculation sheet and chattering.

 

[The Obturator]

AMENDED STATEMENT I think what you mean by '...capacity exceeds 200 - 300 %' is that the PRV selected area is oversized by 33 to 50 %. This is conservative, but in the area considered by many.

I did say earlier approx. 30 to 40 % but it does depend on other conditions including the PRV itself.

Note that ASME limits the lift to a minimum of 30 % (or 0.08" lift min.) but this is not to be taken as the de facto guidance, but probably 30 % is a good figure as a base.

If I were a responsible PRV vendor, for a given PRV application where the calculated area represented 30 to 40 % of the selected area, I would be strongly advising the client that there is a potentially good reason for the PRV to chatter in service. Especially if the service was liquid. However, I would also be advising that other issues may also contribute and/or lead to chatter. I would also be recommending that the PRV has restricted lift (and quote the API/ASME reference) to enable stable operation during opening.

Restricted lift in a spring operated PRV is an easy to do option.

I would also be making some comment for any other calculated areas approaching 70 % of selected area and the availability of restricted lift, especially on larger size orifice sizes where the increase in area between sizes is much greater.

IMPORTANT: These are my own viewpoints from experience - vendors may have their own and differing, ideas and philosophy on oversized PRV's and their limits leading to PRV chatter.

Again, it is not just oversizing, but the whole installation surrounding it.

Hope that helped you more.

I'm sure some other PRV people will come back with their comments which will not be too disimilar.


Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[georgeverghese]

In actual operations, there is no way you can prevent chatter, since the operating relief load in a failure incident could be anything from 0-100% of calculated max relief load. Mechanical damage to the PSV resulting from chatter at low relief loads is minimised by action of other instrumented safety loops that isolate the source of relief or cancel out the relief scenario.
It is good design practice however, to select a PSV size that matches the max relief load with as little excess relief capacity as possible.

Some OpCos have specific guidelines on when to split the design calculated RV load to multiple PSVs' with staggered setpoints to (a) reduce the risk of mechanical damage to the higher SP large PSV, nozzles and piping due chatter at low flows AND (ii) to reduce the likelihood of triggering the larger, higher SP PSV by first triggering a smaller PSV with lower SP. See your company or OpCo design guides for this - I dont recall the specifics of this guideline now. This is usually done for large PSVs ( may be 4P6 ?? and bigger) with high SP. For smaller PSVs' with lower SP, PSV chatter at low flow emergencies is unavoidable.

 

[Sawsan311]

Hi, in addition to what is highlighted above, I would like to add the following:

- all liquid PRVs including the certified ones as per ASME SEC VIII code are prone to chattering and can be mitigated by installing vibration dampner trims. The liquid unlike gas doesn't have the expansive forces to accumulate in the huddling chamber of the PSV and hence due to greater density and momentum tends to induce greater reaction forces when the PRV is lifted and opened. Other effects associated with acoustic interaction explained in API 520 P II are relevant to chattering and can be studied in details taking into account the PRV size and the opening time.
- The typical guide for avoiding oversized PSV induced chattering is to have the capacity above 25% of the rated capacity as explained in API 521 section 5.8.2.2.
- the requirement to meet the 3% maximum non recoverable inlet pressure loss doesn't guarantee NO chattering, this requirement can help in preventing the pressure at the inlet of the PSV throat from falling below the pressure kinetic force required for full lift. Essentially this requirement emerged back in 1948 where it was originally based on blowdown of 5% and was based on reduced bore inlet PSV valves. The allowable inlet pressure loss shall always be compared to the blowdown of the PSV. As per ASME SEC VIII, the differential to set pressure (blowdown) for vapor PSV is 7% for pressure above 69 bars, the inlet pressure loss criteria can be increased beyond 3% (based on PSV rated capacity) as long as the pressure is always above the closing pressure based on the blowdown. If the criteria can't be met, blowdown of PSV can be increased (only possible for spring loaded valves) while taking into account the effect on the overpressure and the prolonged leakage of the process stream to flare as well as reduction in the system's operating pressure.

- Pilot operated PSV don't eliminate the 3% inlet pressure loss limitation. Only non-flowing POPSV equipped with blowdown seat which prevents the flow of the fluid through the pilot when it opens the main valve - don't require meeting the 3% criteria for its remote sensing line. Flowing type POSV still require this criteria to be met.

I would like to enquire about the restricted lift option, can this be considered a temporary solution for any chattering situation? would it be possible to reverse back to the PSV anytime and when would that be envisaged? in my opinion the problem mainly to the rated capacity of the PSV at full lift unless PSV set point increases allowing more PSV inlet pressure losses with no consequent higher built-up backpressure on the valve.

 

[don1980]

I would like to enquire about the restricted lift option, can this be considered a temporary solution for any chattering situation?

Implementing restricted lift (RL) may solve a chattering problem, but whether it does or not will depend on the reason for the chattering in the first place. There are multiple causes of chatter, and without experimental testing of the installation, one can never be certain which potential cause is the true cause.

Implementing RL is beneficial in reducing the risk of chatter because it increases the net lifting force on the PSV. It does that by reducing the pressure loss in the pipe, thus creating a greater dP across the PSV itself. So, yes, it's a reasonable/acceptable modification for an existing PSV that's known to chatter, or for a new installation that has excessive piping pressure losses. Effectively, RL reduces the orifice area of the PSV, thus resulting in a lower rated capacity for that valve. Since pressure loss increases square to the flow, a small reduction in rated flow results in a relatively large reduction in pressure loss.

If the chattering is caused by acoustic phenomena then, of course, implementing RL may not solve the problem. Refer to API 520 Pt II for: (1) the latest knowledge about what causes of chatter, (2) an explanation on how to assess the risk of chatter, and (3) design measures which can reduce that risk.

And, BTW, there's no reason to regard RL as a "temporary" fix. When used, it's generally done as a "permanent" part of the relief design. If one wants to reverse an RL modification, that's easy to do. Just send the PSV to the valve shop and have the RL (mechanical stop) removed.