U-tube press. relief what is flow steam 15psig 10" open end pipe? 1

[rikiraph2]

I intend to use a water filled u-tube to as a pressure relief device for a large steam filled vessel. the pipe of water will be 10.5m high to maintain 1.05 barg pressure.

The reason for this is the correct Safety valve for this application has a 8" orifice and is about $40000.

Does anyone know how to calculate the flow from say a 10" pipe 20m long with superheated steam at 15psig and 140degC (280F)at one end and the other end open to atmosphere.?

 

[katmar]

For a 10" Sched 40 pipe my estimate is 10.3 kg/s - once all the water has been blown out. I have no idea how long it would take to get all the water out. I guess it would be quick, but not as quick as a safety valve opening.


Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[vpl]

Rikiraph2

You say you intend to use a water filled u-tube as a pressure relief device because a safety valve is too expensive. Then, after saying you've already made up your mind, you ask how to calculate the flow in a different size pipe.

How expensive will it be if your vessel explodes and you're held liable for not designing a proper relief system? Do you think saying "someone on a free Internet site told me it was ok" will absolve you of responsibility?




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[rikiraph2]

Thankyou for your threads,

I am not trying to do anything that is not by the book, there are a number of pressure relief devices available one of them is a rated pressure relief valve. There is no reason why a tube of water at 10m high is disallowed by the code, to me it is a perfectly accurate and safe pressure relief device.

 

[rikiraph2]

Katmar,

10.3 kg/s is easily adequate and about what I expect for my application.
But how did you work it out? is it a complex calculation for compressible flow?
Also this design will be verified and be part of the risk analysis.

 

[chicopee]

You should understand that the "U" tube is not ASME/NB approved relief device and consequently if the vessel blows up your insurance carrier will not be happy about paying for property damage, compensation and liabilty coverage. If the loss is significant, your company may even close their doors forever which is not unusual.
Instead of one PRV have you looked at more than one.

 

[skearse]

Not sure where your application is located, but in the US statements like this

The reason for this is the correct Safety valve for this application has a 8" orifice and is about $40000.

are the kinds of things trial lawyers love to see in writing-this is what makes the fines go straight to negligent or (probably) willful and ends up making a lot of plantiffs a lot of money-even if nothing goes wrong yo uare now on the record as having put a price on safety...

Yes, that is a large PSV but depending on the jursidiction you may not have too many other options-the rules for ASME all but require it, and the risk of not having one, to me, far outweighs the benefits of installing a properly sized relief system that meets ASME requirements. As chicopee stated, this soulds like an application where multiple valves are the best option. At the very least, your maintenance crew will thank you for not having to manhandle a huge relief valve. Rutpure disks may also be an option for such a large relief area.

 

[katmar]

Regarding the sizing
Any standard fluid mechanics handbook will cover this calculation. As a chemical engineer I would go to Perry's Chemical Engineers' Handbook or to Crane's TP 410 - check with your chemical engineering colleagues. Googling "isothermal compressible flow" will get you a gazillion hits. It could be argued that your application is closer to isentropic than isothermal, but isothermal will be the more conservative (lower flow) scenario. Remember to include the exit losses.

Regarding the Risk and Code Compliance
I have seen exactly this solution used by a company that I would regard as highly responsible, but as I was not involved with the calculations or certification myself I cannot be sure that they were in compliance. As the others have pointed out, there could be some personal risk to yourself. But you have said that you will get the design verified - just make sure that it is by a licensed engineer and not by anonymous me. I am not an expert in the pressure vessel codes, and they vary from area to area, so make sure you are covered.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[rikiraph2]

Thankyou to all for your replies and concerns,
I figured it out using the method in Perrys as per Katmar's response.
The vessel is not designed to ASME. and the method of using an open vent with water trap as a form of relief is specifically allowed by the local code, perhaps in response to skearse a better wording would have been a spring or pilot operated safety valve is is cost and size. prohibitive.

 

[racookpe1978]

So, over the years, who is going to check (daily? hourly?) the water level in the u-tube that it doesn't evaporate out = no resistance,
flood up higher due to rain or snow = too much resistance,
plug up from debris or dirt,
freeze,
foul and corrrode.

No. Don't do it.

 

[MJCronin]

Riki.....

The comments made above regarding eventual eventual evaporation, tube corrosion and blockage are valid. Especially with a boiler that is at this temperature

Some questions:

If you can afford the boiler, why cant you afford a certified, proper safety valve..??? Contrary to your bosses opinion, this is not a luxury but a necessity.

Additionally, have you considered reused (but but properly certified) safety valves ???

How about a certified rupture disc ?

-MJC

 

[katmar]

Why is everyone so terrified of this U-tube? I agree - don't use it if it does not meet the code. But all the other FUD mongering is not helping. Relief valves also corrode and block and freeze and need testing and calibration. The U-tube can be set far more accurately than a relief valve can. A simple rotameter controlling a trickle of water into the U-tube with an overflow at the correct height is all you need. Or put the flow on a readout in the control room if operators will not be around to see the overflow. If it is legally acceptable, I have no problems with the reliability. As I mentioned earlier - I have seen one working.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com