Excess flow valve questions 1

[mikeg8]

I've been asked to look at using excess flow valves placed into lines before pressure gauges (Bourdon tube type). The idea is that IF the gauge fails, then the valve will close. These lines are really small, 3/8" or 1/4". I don't think it will work. Anybody any relevant experience? Anybody offer an alternative solution that will work?

Mike

 

[Ashereng]

Mike,

You have excess flow valves, say globes, v-balls, etc. You want to used them as root valves or gauge vales, off the line, in front of a PG?

How big are the flow valves. We often use 2" balls and gates as the root valve (considered part of the linne), and then either needle valves or gauge valves (eg CM29's) after them.

The problem may be that these excess flow valves leak. In that case, you still may want another valve before your PG. In that case, you may be further ahead with a new root/gauge valve.

I don't work or benefit from Century Valve. I do spec them though.

Take a look at Century Valve. They have a lot of combination root/guage, block and bleed, double block and bleed, needle valves, etc. The prices are very reasonable. A CM29 is about $130 CDN.

 

[Joesteam]

They do make flow stop valves but not industrial grade that I have seen. If it is a critical application, why not use a diaphragm seal for the gauge. These are generally used in pharmaceutical, culinary, and hazardous material applications, but will work to give you an extra piece of protection between the guage and the fluid in the piping.
Look at:

http://www.ashcroft.com/products.cfm?doc_id=754

 

[Ashereng]

They do make flow stop valves but not industrial grade that I have seen.

Joe,

Who is they? And what is the definition of industrial grade?


With respect to protection for the gauge (diaphragm seal), the purpose of the root/gauge valve is not protection. It is for isolation so that you can remove the gauge without shutting down the line.

 

[Joesteam]

The original post is for a valve that will close if the gauge fails. I assumed that they wanted the valve to AUTOMATICALLY close should the bourdon tube fail. For a manual valve, you can use any valve that is rated for the pressure, temperature, and material in the line. We often use needle valves for this application and sometimes even gas cocks.

The non-industrial type valves are often used on the water supply to washing machines and hot water heaters that automatically close when there is a leak.

 

[Ashereng]

Joe,

The trend here is to replace the needle valves with a gauge valve, such as a CM29 which has integral block and bleed. Of course, there are many other manufacturer.

The savings is in the costs of the time to tube the 2 needle valves up (for block and bleed), and in some cases, the CM29 is cheaper than 2 needle valves.

 

[mikeg8]

I don't think I was very clear to begin with; let me re-state: the pressure gauge is mounted on a 1/4" tube line running hydrogen at around 15barg. There is a double block and bleed assembly before the gauge. Should the gauge fail to atmosphere, we'd like to have an 'excess flow' type valve in the gauge line which will AUTOMATICALLY close so we don't dump too much H2 into the air.

Joe: I had a quick look at the Ashcroft site, but have to confess that I still don't know what a diaphragm seal thing is or what it does!

Thanks for the replies so far guys

Mike

 

[Ashereng]

mikeg8,

Yes, a clearer explanation would have helped. Joesteam evidently read you OP correctly, and I didn't.

A diaphragm seal acts as a barrier between the process fluid and the instrument, in this case, your pressure gauge. For hydrogen service, I have heard that because H2 is such a small molecule, you get hydrogen migration across the diaphragm since the molecule can sort of squeeze its way across the material at a molecular leve. The solution that my boss recommended was a gold (yes, that gold) plated diaphragm seal. Evidently, H2 does not migrate across gold.

I am still a bit unclear by what you mean by a "excess flow" valve. If the tubing is 1/4", how big is this excess flow valve? I have not seen very many 1/4" valve automated. Then again, I do not have too much experience with hydrogen service.

With regards to the gauge faililng to atmosphere, how will you know when the gauge is failing to atmosphere?

 

[mikeg8]

Hi Ashereng, thanks for the reply. I have now solved the problem by using a Swagelok item which they call an excess flow valve (though I still have a couple of questions to ask them). Regarding the valve operation, it would appear to work through spring pressure, rather like a poorly made check valve inserted into the line the wrong way around: a certain flow moves the poppet a little and a greatly increased flow moves the poppet to it's closed position.

You're right, hydrogen is a tricky gas as the molecular size means it can go pretty much where it likes...and we'd like to keep it all in one place! I have been told about the gold plating...and the cost too: my cardiac consultant says I'll be back at work soon!

Thanks all for your assistance.

 

[Ashereng]

Ahhh! Got to get my brain out of MY comfort zone.

I think you are refering to a pneumatic switch and/or pilot to activate the valve. That would work very well also.

In my industry, we instrument everything and use powered devices. The pneumatic controllers, swithes, relays, is something I do not used everyday, and hence have banished to the back of my mind.

Hope your cardiac problem improves. Wait till you get a load of the costs at larger sizes. ;-)

 

[ApC2Kp]

mikeg8,

Excess flow valves do have a spring that is calibrated to allow the valve disk to close with a calculated pressure drop that corresponds to a specified maximum flow.

Remember that the pressure drop is related to flow by mass, velocity and specific gravity (density). The vendor needs to adjust the valve maximum flow rating for the density of your gas - hydrogen. The valve may not trip shut with your hydrogen gas unless a lighter spring is used. For your gage blowout application the design flow could be so small that the spring force would be negligible. The weight of disk could be enough for a vertical upflow condition to push the disk to shutoff position. Repeat, if the spring force is too much, the excess flow valve might not trip shut with the lighter density hydrogen gas.

There was a case of an excess flow valve being applied in a freon unloading line. The excess flow valve kept tripping shut at much lower flows than expected. Finally, someone realized that the excess flow valve was originally rated for LPG, and the freon had a much higher density causing more pressure drop for the same volumetric flow. The actual pressure drop with freon was several times that of LPG at the same flow rate.

Your 15 bar pressure makes the gas density close to that of atmospheric air. You might want to consider a functional test of the excess flow valve with your hydrogen gas and 15 bar condition.

One more caution. Most all excess flow valves have a small bleed port that allows pressure to equalize after the system has been shut in. The spring returns the disk to the open position. You might need to consider the continuing leakage of hydrogen (small or not so small leak) after the excess flow valve has tripped "shut". The valve vendor may have option of "no bleed port", and you could take advantage of the hydrogen gas ability to leak past the valve disk for pressure equalization.

 

[Joesteam]

This is excellent information!!

 

[JimCasey]

An excess flow check (different name, same idea) will stop the flow when it exceeds a certain value. Unless somebody runs over the gage with a forklift and snaps it off at the socket it would seem unlikely that it would fail catastrophically. More likely it would corrode or fatigue and begin leaking slowly.

My fearless opinion is that you would be better served spending your money on flammable gas detectors (Scott, MSA, Etc.) and sticking them up against the ceiling where the H2 will accumulate if there is any.

 

[EGT01]

I'm thinking more along the lines of JimCasey and don't know how much faith I would put in an excess flow valve (unattended). If you have concerns about the gauge failing, why not make use of your block valves on your gauge connection. Develop an operating procedure that requires the block valves to be closed after taking your pressure reading.

 

[sairaj]

since the lines are smaller in sizes with instrument nature,the possibility of upgrading the sizes are less likely,instead why dont you think about solenoid valves with near by utility air loop for its operating