WATER SAND FILTER SAFETY VALVE 3

[roker]

HELLO,

A water sand filter was designed acc to ASME, design pressure is 7.5 barg.
do we need to install a safety valve on this vessel?
if yes, to what case it should be designed? for blocked outlet the vessel is protected,
should we consider a fire? because it is a water plant there is not cause for a fire to occure.

regards,
roker

 

[bimr]

If the tank design pressure is greater than the pressure head (dead head pressure) that the water supply pump puts out, then no, you do not need a relief valve.

I have never heard of someone considering a fire scenario for a water plant.

If you have a fire scenario or the vessel will be exposed to the sun, you may consider using a 1" sentinel relief valve.

 

[roker]

bimr,

thank you, but if we didn't hear of something it does not mean that it don't exist, what about water plants that are near steam boilers? could you advise what ASME requires?

thank you again and regards,
roker

 

[sbush]

The ASME code is crystal clear on this subject. All unfired pressure vessels, such as the one you described, must be fitted with an ASME rated pressure relief valve. They must be sized in such a way that the maximum allowable working pressure (MAWP) of the vessel cannot be exceeded (regardless of whether anyone thinks it could ever happen or not). As far as sizing, you should consult with a manufacturer of ASME rated safety valves before you make a purchase. Knuckle Valve is one that is knowledgeable and respected in this area. In addition to the MAWP of the vessel, you will need to establish the maximum flow rate that your system is capable of supplying to the filter at any given time. Usually, the relief pressure set point is set for 10 percent below the MAWP, but this isn't always the case depending upon the manfacturer's recommendation based on the precision of their valves.

The pressure relief valve can be located anywhere on the vessel or piping attached to the vessel; however, it cannot have any device such as a shutoff valve between it and the vessel that could isolate it from the vessel.

You do not have to take a fire into consideration. For your application, the safety valve should be designed for water, not steam.

S. Bush

http://www.water-eg.com

 

[jte]

As SBush noted, ASME VIII-1 is pretty clear on requiring all U stamped vessels to have a relief device as stated in UG-125: All pressure vessels within the Scope of this Division, irrespective of size or pressure, shall be provided with pressure relief devices...The exception to this rule (remember, ASME is Always, Sometimes, Maybe, Except)is the code case which allows overpressure protection thru system design. This is a topic which has been discussed previously on eng-tips; a search should easily turn up the previous threads.

I'll take one exception to one comment which SBush made: Valves are allowed between the relief device and the vessel per UG-135(d) and, in fact, this is common practice in my experience. What I've seen is that block valves leading to relief valves are CSO'd (car sealed open) which provides an administrative control which meets the requirements of UG-135(d) and the local Jurisdictional regulations also.

Which brings up my final comment: Check with the local jurisdiction!

jt

 

[TD2K]

I've been in many refineries and processing plants.

Rarely have I have seen a PSV without a block valve under it or on the discharge if it's going to a closed collection system like a flare. Why? Because if you need to remove it for maintenance, you aren't going to design it so that the entire processing unit has to be shutdown because a PSV has to be removed for testing or is leaking. Usually for this reason two 100% PSVs are provided each with their inlet and outlet block valves but that's not the case 100% of the time.

In some cases, the inlet valves are three way valves that ensures you never shut off the inlet to one PSV before the standby PSV is 'on-line' but those are relatively rare setups in my experience.

If I can't come up with a credible relieving case and the client or jurisdiction wants a PSV in any event, I will typically propose a thermal relief valve. Those are typically a nominal 3/4" by 3/4" with a minimum orifice and typically handle most thermal relief requirements (not including a fire of course).

 

[bimr]

Some things in this thread have not been crystal clear.

Here are some articles that may be of assistance:

http://www.chemicalprocessing.com/articles/2001/324.html

http://www.cheresources.com/asiseeit1.shtml

The first thing that you have to establish is the MAWP. The MAWP should be stamped on an ASME Section 8 tank.

Then you need to analyze your system. If your pumps have the capability to raise the system pressure above the MAWP, then you need a relief valve. If the pumps do not have the capability to raise the pressure above the MAWP, then you do not need a relief valve because the Code allows overpressure protection thru system design. This overpressure protection thru system design scenario is quite common for water treatment equipment.

If you need a relief valve, then the relief valve should be set at the MAWP, not 10% below as suggested by SBUSH. The relief valve should also be designed to ASME Section 8 requirements.

I have never seen an isolation valve on a water filter tank safety relief valve. The reason being is that there are usually multiple water filters and it is quite easy to isolate one filter tank from service to check the relief valves. Water filters are also not a critical system since there is usually a spare, or there is overcapacity, or the other filters can be operated at a higher capacity for a short duration.

Contrast that to a refinery situation where you have to shut down the entire process unit to check a safety relief valve. That is where you want the isolation valve. It is not the same scenario with a water filter.

The final thing that you need to address is to evaluate thermal expansion. Overpressurization resulting from thermal expansion is unlikely in a water treatment system, but should be evaluated.

"However, for low- to moderate-pressure systems located outside in direct sunlight, with pipe runs greater than 100 feet and operating as a batch system (often isolated), engineers should provide PRVs in piping systems if the fluid has a low boiling point (i.e., is cryogenic), regardless of pipe length; is a hazardous fluid; or is part of a large-volume system."

 

[Ashereng]

If your pumps have the capability to raise the system pressure above the MAWP, then you need a relief valve. If the pumps do not have the capability to raise the pressure above the MAWP, then you do not need a relief valve because the Code allows overpressure protection thru system design.

bimr is correct I believe. I have also heard that if the vessel is rated to a higher pressure than the maximum pressure possible in the system, a PSV or other relieve devices is not required.

I will have to double check this, and post if I am mistaken.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[epoisses]

I'm not a safety engineer so this might be a silly question...:

Isn't there a certain minimum size below which a vessle does not need a PRV? I am saying this because I recall a minimum diameter of I believe 24", but don't know whether this was an ASME or an other requirement.

 

[bimr]

It may be more economical to use a 24" pipe instead of fabricating a 24" dia. vessel. But, piping must be protected with relief valves as well. So the pressure relief protection will be governed by the piping code instead of the vessel code.

 

[pleckner]

A. This vessel may not necessarily fall within ASME Section VIII, Div. 1 scope as given in the scope document, secton U-1. The exclusion from ASME scope is defined in paragraph U-1(c)(2)(f): a vessel for containing water under pressure including those containing air the compression of which serves only as a cushion, when none of the following limitations are exceeded:
(1) a design pressure of 300 psi (2 MPa);
(2) a design temperature of 210 F (99 C)

The second exclusion that "epoisses" was trying to address comes from paragraph U-1(c)(2)(i): vessels having an inside diameter, width, height or cross section diagonal not exceeding 6 inches (152 mm), with no limitation on length of vessel or pressure.

It doesn't appear that this vessel meets these exclusions. First I get the impression it is bigger than a bread box. Second, even if it is not included in the ASME scope, it still can be designed to ASME and if it is stamped, it must be protected and this appears to be implied in the original post.

B. If there are no credible scenarios for overpressure identified, you can indeed invoke CODE CASE 2211 that allows containment by design. BUT, the regulating authority in the jurisdiction must pre-approve this. You cannot invoke this CODE CASE by simply saying you want to!

C. For a credible fire scenario, the recommended procedure is to consider a fire zone of about a 50 foot diameter circle. This means if this vessel is located within a 50 foot diameter circle of a another vessel that can initiate a fire, then a fire case would be credible.

D. In the absence of a credible relieving scenario, I do what "TD2K" does, put in a minimal PSV for thermal relief.

E. I have to second the disagreement with "sbush". You may set the PSV right up to the MAWP, period. You can have isolation between the vessel and the PSV as long as it is car-sealed opened.

F. Note that PSV manufacturers won't "size" the valve for you; it's a liability issue. You must give them the required flow and conditions. They will "design" the valve.

G. Location of the PSV can be almost anywhere but it is highly recommended that a PSV relieve liquid primarily should be put in the area where it is always in liquid and not in a vapor space. Most of us in chemical/petrochemial business have actually been doing this wrong for the most part.

 

[roker]

hello all,

I didn't realize it is an issue so many are dealing with.

thanks and regards to all of you,
roker

 

[sbush]

My eighteen year old son inherited one of my old cars. All he has to do is keep gas in it. I take care of the rest. It's a fairly sophisticated car, albeit a little aged. It has a digital display for reading the vital operating parameters such as MPG averaging and miles remaining until empty.

So one day I'm riding with my son. We're cruising along and I notice that the arrow on the gas gauge is dead on empty. In an alarmed voice, I tell him that we're traveling on empty! To which he calmly reaches up to his trusty digital display and pushes a few buttons. No, he replies. We've got six miles 'til empty.

Roker,

It always pays to play it safer and, fortunately, the Code allows you the latitude to do so.

As JTE suggested, get advice from your local boiler & pressure vessel inspector, or the appropriate insurance entity that covers this area of your plant. If you decide to install a pressure relief valve, try to build-in a safety factor (especially if you already have ample pressure) by lowering the set point. And, as Bimr pointed out, don't install an isolation valve if you can isolate the entire filter unit in order to service the safety valve.

S. Bush

http://www.water-eg.com

 

[pleckner]

IF this is an ASME coded vessel and the relief valve used is ASME certified (which is needs to be), then I'm just going to have to agree to disagree with 'sbush'. You do not need any margin between the vessel design pressure (or MAWP) and the PSV set point. You can and it is your choice to do so but it is not necessary. In the chemical/petrochemical/pharmacetutical industries, we deal with high pressure vessels all the time and many contain or treat some pretty nasty stuff. We typically do not put margins between the PSV set point and the vessel's design pressure (or MAWP).

As far as isolation valves between the vessel and the PSV, again, it is your choice but there is nothing that should stop you from doing so if you so wish...as long as it is locked opened and cannot be closed without proper authorization and supervision. I would agree with 'sbush' that if you don't need it, don't put it in. It does become just another leak point and an over all pain.

The rules were set up to promote and enhance safety, not prohibit operation. The rules in of themselves are quite conservative. Follow the rules and you will be safe without having to "over do it".