Pressure Drop-PSV Inlet 2

[RJB32482]

I have a question about adding ball valves to a pressure safety valve for our water/propylene glycol mixture (reactor jacket cooling/heating). The ball valves are to be added on both sides of the PSV so it will be easier to take out and perform PM's on.
Now adding these valves would increase the pressure drop on both sides of the valve. My question is what will the effect of this pressure drop be. My thoughts were:

1. Could cause backflow is pressure drop is greater than pressure in reactor jacket.
2. Would decrease the pressure right before the PSV, effecting how it will open (won't open until higher pressures in the jacket occur).

Any other thoughts and suggestions? Are my points valid?

Thanks Everyone

 

[EGT01]

RJB32482,

At the risk of sounding like I'm criticizing, your lack of experience is quite obvious. Your quest for knowledge is commendable but I would strongly suggest you seek out a senior associate to guide you in this task. Relief system design is not something you can readily learn from an internet forum.

You are on the right track recognizing that the block valves will increase pressure drop and this must be properly accounted for. However, regarding your first thought, if you give it a little more thought, I think you will see it is totally counterintuitive. Flow only occurs from points of higher pressure to points of lower pressure. The only way you will get backflow through your reactor jacket relief system is if you connected it to a downstream point that has a pressure greater than what your jacket is allowed to have and that would make no sense.

Regarding your second thought, as long as the inlet block valve is open, the block valve causes no pressure drop unless there is flow. Therefore, the block valve has no effect on the opening pressure of the relief valve because there is no flow until after the relief valve opens. Now, once the relief valve opens, the block valve will cause additional pressure drop that may cause the pressure at the relief valve to fall below its blowdown point. This will cause the relief valve to close, obviously something you don't want to happen during an overpressure event. Excessive pressure drop in the inlet or outlet of a relief valve will result in reduced capacity of the relief system and can cause a condition called chattering that can have detrimental effects on the relief valve and possibly the piping/equipment to which it is connected.

Again, this is not a task for an inexperienced engineer and you should have a good understanding of industry practices and codes before proceeding. You will find a discussion about block valves for relief systems in API RP-520 Part II and in ASME Section VIII, Division 1, UG-135 and Appendix M. However, these references are only a small part of the total that you need to properly design your relief system.

 

[jsummerfield]

If adding ball valves to a pressure safety valve assure that they are full-port. If the valve size is larger than the PSV inlet size check the full-open diameter of the ball-valve. Assure that block valves are locked or sealed open.

John

 

[Montemayor]

RJB32482:

From another ChemE, pay particular attention and heed what EGT01 has correctly pointed out in great detail. These are words of wisdon and worthwhile heeding - not only for your personal improvement, but for your (& others) safety in the workplace.

I also applaud your recognition of the importance of the 10% pressure drop ruling in the PSV inlet nozzle. However, speaking with many years of experience, this is not a big challenge when compared with knowing all the other vital factors surrounding the application of a PSV correctly and safely. You can easily overcome the inlet nozzle pressure drop constraint with today's full-bore ball valves or even an oversized nozzle and valve.

Take it from me, EGT01's comments - while maybe not to your satisfaction - are all positively pointed in your favor and for your continued success. He has done you a great favor in carefully analyzing your real need in depth and perception.

 

[RJB32482]

I want to thank everyone for their responses. Yes, I am an entry level engineer who just graduated college. I understand how important the safety of the plant and other is. A more experienced engineer will perform the calculations. I was just trying to gain knowledge from other engineers with more experience in valve calculations. I will take everyone's advice to heart and thanks again for the helpful input.

 

[unclesyd]

You had better check with your insurance carrier. Our standards and insurance carrier would never stand for a block valve of any description in a PSV inlet line or vent line on any system.

The only type valve permitted was like the following.

http://www.andersongreenwood.com/safety.asp

 

[RJB32482]

The ball valves are going to be car sealed open, so there should be no problem with the valves closing unless we need maintenance on the PSV's.

 

[RJB32482]

jsummerfield, what do you mean about checking the full-open diameter of the ball valve if its bigger than the PSV inlet. Is there a rule of thumb or standard for this?

Thanks

 

[MortenA]

car sealing is a waste of time and money.

Either you lock them or you leave them.

The car seal is just false security. Over time someone is just as likely to forget resetting the seal as to forget opening the valve.

Just my personal opinion.

Best regards

Morten

 

[Latexman]

With the proper vision, message, procedures, training and work rules (for everyone - operations, maintenance, construction, and staff), a car seal is statistically almost as good as chains and locks without the hassle of the keys. Chains and locks provide added protection against dishonest employees. My personal experience.

Good luck,
Latexman

 

[jsummerfield]

The typical ball valve is a reduced port valve. The ball is about a line size smaller than the valve size. A full-port valve has a line sized bore.

Let's assume a 4X6 PSV. Perhaps the line upstream of the PSV is 6-inch with the 6-inch ball then a reducer and the PSV. In this case, assure that the 6-inch ball valve internal diameter is as large or larger than the 4-inch line.

I too prefer locks over car-seals for most locations. The operating superintendent can have greater control over keys. However, recently I worked offshore where issues may exist regarding dropping keys through the grating. I can be flexible in this regard. However, differentiate car-seals from tie-wraps that operators often use these days for various commissioning and startup or maintenance activities.

John

 

[roker]

hello,

according to API 520/521 the pressure drop in the inlet line to the PSV shall not be greater than 3% of the PSV set pressure, at the outlet line the pressure drop allowed depends on the PSV type with/without bellows, pilot, etc.

regards