why slam shut could close? 3

[Gasdealer]

in our network of natural gas. 4 of our slam shut in 4 diff. regulators got closed in the same time they are in the same loop. and we don't have a reason of the increase of the pressure which cause the closure!!
we tested the settings and it was ok.
any advice?

 

[BigInch]

Same control loop or same pipe loop?



BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[gerhardl]

Temperature increase excluded as possible cause? More details necessary!

 

[Gasdealer]

Same pipe loop,
about the temperature , it was at night so temperature was low already
Thanks

 

[BigInch]

This could go faster. Write down everything you know about the prevailing conditions at the time.

pressure spike?

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Gasdealer]

the temprature was around 15 celisus working pressure is 100 mlbar slam shut at 135 mlbar
it's a polyethelene domestic network in the city itself.

 

[JimCasey]

In the same loop, if one slammed it would likely cause a pressure spike to affect the others. Even if it only affected one other, when the second one slammed the spike could cascade to the third, etc.

Are they near a large user who could have had a sudden load change?

 

[vzeos]

Gasdealer,

The purpose of a slam shut is to protect the piping down stream of the regulator. I assume you have 4 regulators feeding into one system and that each regulator is protected by a slam shut. Now, during midnight, the load is at minimum and the pressure distribution is flat. If, under low flow conditions, a regulator has some dirt in the seat, gas will continue to flow into your system and the pressure will build. Therefore the back pressure coming from one defective regulator can cause all of the slam shuts to activate. You should test your regulators for a tight shut-off.

 

[Gasdealer]

thank you all,
please Vzeos !!
you said (a regulator has some dirt in the seat)
you mean it's filter?? or the regulator itself?
because the filter was clean.
the regulator is new so how can i prevent this from happening again?
THanks

 

[vzeos]

Gasdealer,

I mean the regulator itself...When I say dirt in the seat, this is a figure of speech meaning anything that prevents the regulator from shutting off tightly. It could be dirt, rust, mechanical defects or any bug (not a literal bug)... Also, you said you have 4 regulators feeding the system. If back pressure is indeed your problem, the back pressure could be coming from any one of 4 regulators. It is not sufficient to say that one regulator is new and clean. It might be a good idea to do a lock-up test on each regulator feeding the system to see if you are getting a tight shut-off on each.

 

[Gasdealer]

Hello,
how can we make this lock-up test?
also this problem happened again.
and we got to purge some points in the line. and it show that the line have some water and air!!!!
i think the commisioning wasn't good at all.
do you think this is the reason? and if yes? how i will cause the pressure increase?

 

[vzeos]

Gasdealer,

First of all, you should get a qualified professional on site as soon as possible. Trouble shooting a natural gas distribution system is serious business and really should not be done in an engineering forum.

Your qualified professional can perform a lock-up test by installing a pressure gage between the outlet of the regulator and the station outlet valve, and then closing the station outlet valve. If the gage pressure increases above the regulator set point, then the regulator is not shutting off completely and it needs to be serviced.

You must check all regulators feeding the system but do only one regulator at a time. Before you shut down any stations, make sure there is enough regulator capacity that will allow you to take one regulator out of service and still supply your customers.

Water can cause control problems in regulators as well as pilot regulators. How this happens depends on the type of regulator and where the water gets trapped or accumulates.

Get a qualified professional on site as soon as possible.

 

[Gasdealer]

we have qualified professional working in this matter, but i was curious, how water or Air could cause pressure increase in the network

 

[Gasdealer]

one of the regulators caused it.
THanks for your help