In-situ Vs Bench testing results

[Seba93]

Hi Guys - I have two questions about PSVs and the two testing methods i.e. in-situ and bench testing.

1. If a PSV fails an In-situ testing, would the results be accurate enough as compared to bench testing results; especially with set pressure and CDTP (Cold differential test pressure)?

2. If a PSV was tested in-situ after a very high vibration situation, would the test results be accurate enough? especially if it failed over 110% of CDTP

 

[The Obturator]

Let me start on what may well be a series of comments.

1. If a PSV fails on in-situ testing, depending on why/what failed, it would require removal and attention on a test bench. You would then check the set pressure before before disassembly and compare to the in situ result.

2. Again, unknown level of damage if any. Caution may be necessary if the valve is damaged in some way (guiding, spindle,, seta etc.), which in situ cannot determine, that would prevent it going into full lift (ie, 110%). In situ generally does not open the SRV fully.

The thing with in situ testing, is that certain elements of the SRV must be known. In particular the "pressure area" of the seating (measured in between the inner and outer diameters of the nozzle seat). Some users determine this from doing an in situ test before process is started and calculate it back. Some have the manufacturers measurements (but with tolerances), some (best) get the manufacturers to record the actual measures on the test cert.

Despite what in situ vendors sell, you cannot accurately measure criteria like full lift and blowdown/reseating, since you do not have the process media flow acting physically against the valve. In situ has often contributed to damaging an SRV by activating set pressure at less than 75% system pressure (like a lever requirement), which stretches and damages the spindle.

Hope that's some food for thought.

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

 

[Seba93]

Thanks Avalveman
The scenario is that we have never had very high failure rates of PSVs in our plant. Unfortunately after a very high magnitude earthquake, we have seen an increased number of failures.

Would the type matter? i.e. Conventional, Bellows, Pilot with regards to vibration? Misalignment maybe?
Are there any books / standards out there about valves that give a good view of failures of PSVs/SRVs due to vibration and what to expect when SRVs fail an in-line testing after an event of high vibration?

If its not the vibration, what else should I look into? Service maybe?

 

[The Obturator]

Hi again Seba93,

I do not know of your process, installation or location and cannot comment fully. What I can advise you is that severe vibration would require some form of SRV review. Threaded components could be shook loose (valve cap, body to bonnet fasteners etc), not fully but enough to cause a leakage if it is a closed system. Bellows constructions could also have broke (durability of bellows varies widely between manufacturers). Set pressure settings easily knocked out of tolerance due to misalignment etc. Such observations should not be limited to just the SRV's but the entire system. I cannot think of any written recommendations from other users or experience. However some bona fide SRV manufacturers supply SRV's to Nuclear specifications (ASME III) and part of that design relates to seismic effects. Approaching them may be an idea and/or other users.

My recommendation. Remove the SRV's and check fully;
i) Visually for any obvious defect/loose parts (cap, bolts, nozzle, if any bellows fragments in outlet, if bellows intact.
(for pilot operated types the tubing and pilot mounting will be at real risk).
ii) Perform set pressure and leakage test. Are the results acceptable?
iii) Carry out shell pneumatic test to check joint integrity (and also if bellows is pressure tight between body and bonnet).
iv) Strip SRV. Check all guiding components, seat condition, integrity of spring, trueness of spindle.

These are just basic generic guidelines. Use the manufacturers instructions. Performing such exercises will in time allow you the user to build up a portfolio of observed maintenance procedures based on your plant and determine period of overhaul etc.

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

 

[Seba93]

Thanks again Avalveman!
I am in the oil & gas industry, we deal mostly with LNG Gas so technically the systems are supposedly clean.

Most of our Pilot Operated SRVs have failed a leak test compared to Bellows and Conventional type (which have failed either due to leakage or did not "pop" or relieve at the required pressure, probably jammed due to misalignment).

I have been advised by senior valve techs that it is expected of a pilot operated SRV to fail a leak test when there is high vibration and for bellows and conventional SRVs to be jammed due to their designs. in your line of expertise, do you think this is true? If it is, would you explain to me how / why this is so?

If I were to do an analysis of SRV failure rates for pre and post earthquake; what would you recommend I look into to understand why more SRVs are failing now compared to before the earthquake in addition to Service (Fluid it operates in)? System? Internals (a must)?

 

[The Obturator]

Hi again.
I'm a little puzzled re the comment that pilot operated SRV's failed leak test. First of all, in theory anyway, they should never leak because of their design. Then again, POSRV design varies greatly between manufacturers. Could it be that the test method used in not in line with recommendations? You will get a POSRV leak greatly (out the main valve outlet)without it reaching set pressure....why? POSRV tightness is effected by the system pressure, where it is concentrated in the area above the main valve disc where the area is larger than the nozzle area. ie., the higher the operating pressure the tighter the valve.

Depending on POSRV design and any added accessories (often adding more plumbing in the form of tubing), POSRV may well be more suspect to failure at higher vibration effects. I do believe some form of dampening may be possible but you need to speak to the vendor on that.

For spring operated conventional and bellows design SRV's. again, depends on the design used. Some brands larger sized SRV's (4" inlet plus, may have large and solid diameter components such as discs and guides. These could possibly cause failure with high vibration by jamming. Some designs do not use full nozzles and have the nozzle swaged in. This can loosen. The list can go on. Modern API SRV designs have smaller diameter top guiding and disc inserts now rather than large and heavy solid parts (imagine a 12"+ diameter solid disc travelling inside a full guide of same internal diameter - for many years a few engineering contractors specified avoiding large orifice spring SRV, with solid discs at high temperatures due to concern of such failure)

If your SRV's are from one manufacturer you best speak to them on failure modes and the recommendations they have regarding your systems.

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