ATM Relief valves.. Vertical.. is a small hole in the drain allowed? 1

[jamesbanda]

Dears Sirs,

On relief valves to atmosphere discharging vertically. Do you know if you are allowed under api to have a small drain hole open all the time to prevent rain accumulation ?

(provided the chemicals at release temp are below autoignition temp).

ive seen this done at a few sites but i'd like to understand if this is specified in an approprate code.

 

[CJKruger]

jamesbanda, these drains are required by the ASME code. The quote below is from UG-135:

"Discharge lines from pressure relief devices shall
be designed to facilitate drainage or shall be fitted with
drains to prevent liquid from lodging in the discharge
side of the pressure relief device, and such lines shall
lead to a safe place of discharge."

 

[JoeWong88]

Drain are required by API. Refer figure 1 in API RP 521, Aug. 2003 Fifth Edition.

JoeWong
Chemical & Process Technology

 

[jamesbanda]

Sorry for not being clear.

Are these drain/lines allowed to be always open (eg no valve just a drain hole)..

Please see the picture

so is case a or case b correct..\

 

[zdas04]

Here is a case where one code can require something while another prohibits it (that is why people hire engineers)--if the required drain hole exhausts too close to an electrical component then the existence of the drain hole can change the electrical area classification and require intrinsically safe (or Class 1 Div 1 rated) electrical components in places where Class 1 Div 2 would have been fine. I've seen that twist cause compressor skids to be shut in for code violations.

Putting a valve on it doesn't help the area classification issue and it can allow the accumulation of liquid in the pipe between drain events (which would probably be pretty infrequent in the real world), so the valve gives you the worst of all possible worlds.

The drain holes are a good idea if you are very careful about where they are located relative to electrical components and other ignition sources.



David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"Everyone is entitled to their own opinion, but not their own facts" Patrick Moynihan

 

[BenThayer]

you might consider weathercaps as a means to prevent rain in the discharge as well.

http://www.weathercap.com/weathercap.htm

 

[jamesbanda]

Ben,

Have you used these? and if so for how long..

 

[don1980]

There's no need for concern about having this weep-hole in the tailpipe, unless you're doing something you shouldn't be doing in the first place like discharging a hazarous liquid up in the air.

When the safety valve opens, your relief fluid won't flow out of this hole. Instead, air is educted INTO the tailpipe.

 

[4Pipes]

Weep holes have been a point of disagreement on every project I have known. I would argue that common sense says:
a) Drainage is required if liquids can build up around the valve. Are code references to "Drain Holes" really only a very poorly worded statement of this intent? I would suggest it is. (I have never come across drain holes on hazardous or flamable duties).
b) Drain holes are dangerous even for "non-hazardous" vapours like steam. (I would not be very happy to stand next to even a large compressed air valve. Bits of crap, loose insulation, cladding etc could be blown in all directions).
c) Liquid from drain holes will probably run along the pipe as well as straight down and could get anywhere. It will corrode and damage equipment below - in particular lagging.

I would argue 2 things
a) Drains are required where liquid can collect but must be piped a short distance to safe place. Maybe simply pointed away from a platform.
b) Drains must be roddable to be able to check that they are not blocked. e.g. Fit unions on all straight legs.

You could always connect the drain to the RV body drain plug. Some folk use it but most argue that the valve drain cannot be used for the pipework. (Including one major manufacturer and several large operators and contractors I know). Does somebody have a reason for this argument? Is it because most folk cannot see the similarity in concept between a "drain hole", a "drain plug" or a "drain branch" and just want to cover their asses from a code wording point of view ???
I have lost this argument on 20 of the last 20 projects I have worked on - because Process Dept have always specified on the P&ID that "Drain Holes" are required !!!???

 

[JoeWong88]

Interesting...you probably a responsible piping engineer...
I have some opinions on your concerns...

a) Drainage is required if liquids can build up around the valve. Are code references to "Drain Holes" really only a very poorly worded statement of this intent? I would suggest it is. (I have never come across drain holes on hazardous or flamable duties).

a) For hazardous and flammable product, they normally will be routed to proper closed disposal system i.e. flare network. They do not discharge locally. Thus, no hole is required.

b) Drain holes are dangerous even for "non-hazardous" vapours like steam. (I would not be very happy to stand next to even a large compressed air valve. Bits of crap, loose insulation, cladding etc could be blown in all directions).

b) Drain hole x-area is rather small compare to PSV outlet line x-area. Thus, fluid i.e. steam (almost all) will flow in the outlet line and minimum via the hole. Hydraulic studies can prove this. Having said that, there is still some fluid passing the hole. Generally a safety analysis (HAZIP/HAZOP...) will be conducted and necessary measures i.e. barrier, extended drain pipe instead of hole only, etc will be provided

c) Liquid from drain holes will probably run along the pipe as well as straight down and could get anywhere. It will corrode and damage equipment below - in particular lagging.

c) The "Liquid" probably is rain water. All piping or equipment will expose to rain water.


For your 2 arguments

a) Drains are required where liquid can collect but must be piped a short distance to safe place. Maybe simply pointed away from a platform.

I do agree with this. As mentioned, normally a safety analysis will be conducted and necessary measure will be provided...

b) Drains must be roddable to be able to check that they are not blocked. e.g. Fit unions on all straight legs."

No doubt is a good idea in case a drain pipe is installed.

You could always connect the drain to the RV body drain plug. Some folk use it but most argue that the valve drain cannot be used for the pipework. (Including one major manufacturer and several large operators and contractors I know). Does somebody have a reason for this argument? Is it because most folk cannot see the similarity in concept between a "drain hole", a "drain plug" or a "drain branch" and just want to cover their asses from a code wording point of view ???

The intention of drain hole is to avoid rain water accumulation and minimize rain water get into the valve. Rain water will flush rust, particle stick on the pipe, etc into the valve and it may increase the possibility of getting into the valve seat. So i think it is not really advisable to use the drain hole on the valve body.

I have lost this argument on 20 of the last 20 projects I have worked on - because Process Dept have always specified on the P&ID that "Drain Holes" are required !!!???

I guess your process engineers are not "stubborn" enough and not listen to any advices and concerns. Proper communication, discussion and understanding may properly helps to clear your doubt why you lost all your 20 "battle".



JoeWong
Chemical & Process Technology