City Gate Station Bypass

[Feedyourhead]

Thought this thread discussing gate station bypasses was interesting.

thread378-465800

So let me ask:

Should natural gas gate stations have a manual or semi-automatic bypass? What sorts of designs would constitute a sound bypass run? A bypass with OPP, no OPP? A manual or automatic throttling valve? What kind of valve?

What are you going to do when the transmission pressure drops to or below the MAOP of your system? Should sound gate station design include some sort of fail-safe mechanism for when the transmission inlet pressure drops too low? What other fail-safe devices could be used except for a bypass? Is on the spot tampering with regulators to reduce the pressure loss across them justified in emergencies? What obligations do engineers have to the public to continue service under such circumstances? Consider that now people are going to freeze if you lose service, which could lead to deaths -- but people also might die if you overpressure the system. Though, downstream district regulator stations are likely to catch any overpressure in the system. Frostbite or hypothermia could become a risk after 12 or 24 hours without heat. Should peakshaving facilities be utilized before attempting to use a bypass run, or should a bypass run be utilized before peakshaving facilities?

What sorts of operational procedures should be written for the safe use of a bypass run?

 

[alchemon]

High pressure bypasses are fairly common on stations, especially if they have a redundant OPP. They should be manually operated and double valved. One valve should be suitable for throttling service (e.g. a plug valve). If space and budget permits, my recommendation would be to include a full sized relief valve downstream of the bypass and protects the downstream from accidental overpressure while bypassed. This would be in addition to a worker-monitor set up on the regulator runs. Their purpose is not really for maintenance, as you should design dual regulator runs, but can allow you to manually bypass the station in the case that the inlet pressure drops below the downstream MAOP. You need to weigh the value of the bypass when there is a large difference in MAOPs across the station, as the risk of the inlet dropping that low may be non existent. However, some operators place them in these scenarios (for example 500 psi inlet 60 psi outlet). As far as whether they meet code, call and ask the PUC their thoughts. I will note I've installed them across the eastern US and never had a negative comment during review.

 

[1503-44]

Totally agree, except for
"in the case that the inlet pressure drops below the downstream MAOP"
Or I did not understand your scenario correctly. You still will not get any more pressure than the inlet pressure going into downstream. In fact you could get backflow from the distribution system into the transmission line. Hopefully the meters have a check valve.

The regulators normally in use should have that pressure relief capability already and the bypass should go to the same relief valve. And as we have learned from the Boston incident, don't ever disconnect any part of that system. Still strangely enough, the pressure relief is not required by the B31.8 code, nor the CFRs. A MAJOR ERROR ... IMO.


--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[alchemon]

What I mean by

in the case that the inlet pressure drops below the downstream MAOP

Say for example, you normally get gas at 500 psi and reduce it to 400 psi. Say on a cold day, the inlet pressure drops to 350 psi, your regulators should close until the outlet pressure is below their set point. The system demand will gradually draw down the outlet pressure until it renormalizes and the regulators open (aka below set point). However, let's say your regulators capacity can't push enough gas at 350 psi inlet. In that case, you might decide to manually bypass them via the bypass. No over pressurization occurs because you are pushing gas at 350 psi into your 400 psi system. This happens all the time at City Gate Station served off certain large nameless interstate pipelines.

RE your last comment, I sleep the best at night with stations employing a worker-monitor and full sized relief. However, the relief is not liked by some operators due to the safety, noise, and now environmental issues. But perhaps seeing and hearing when it goes off isn't too much of a bad thing!

 

[LittleInch]

If the d/s pressure is set to 400 psi, then as your inlet pressure fall below this, they will go full open. For a distribution system where there is constant flow, it is difficult to see how the d/s system can end up with a higher pressure than the inlet. However this might occur if you have check valves installed and the flows are very low, but at some point the d/s pressure will fall.

Regulators don't push anything. They control based on a pressure difference.

If the safety relief valves are going off then something has gone seriously wrong and this sort of failure should not be normalised but treated as a serious incident.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[alchemon]

Yes I agree that it is more of a theoretical situation, i.e. that the inlet could drop below the outlet. Obviously, if there is demand on the system, it would draw the outlet pressure down.

However, the purpose of the bypass is still the same. Which is to say it lets you bypass around the regulators if the inlet drops below the outlet MAOP, which happens fairly frequently at City Gates. I have worked on projects where the upstream MAOP was 800# and downstream MAOP was 400#. The actual inlet pressure would be as low as 300# on a cold day. It varied depending on when the upstream operator ran their compressors or not.

And the push term is more of a cultural semantic way of expressing what a regulator does. You are correct that they don't little push gas say like a prime mover would do.

 

[LittleInch]

Bypassing any regulators is, IMHO, a bad idea and in this case if your regulators are fully open, it would make very little difference in pressure to have a bypass open, maybe 10-20 psi at the most.

It might be semantic and I understand that, but we're engineers and should be speaking in technically correct language.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

I have only bypassed regulactors for the specific case of providing gas for black starts; ie. starting power gen sets of remote pump and compressor stations, or offshore platforms. Strictly supervised situations. Never on a city gate, but for highly specific situations, it could be useful.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[alchemon]

The regulator's capacity, among other things, come into play when the inlet pressure drop. Maybe they are designed to pass X MSCFH at 500 psi. How much can they pass at 300 psi? Also, depending on the design of the station, you may need something like 50 psid to open both regulators. If you are already operating at a reduced pressure, the additional drop is a tough sell to a lot of operators. A very popular boot style regulator as a worker monitor needs something like 50 psid across the system to open.

If you don't like manual bypasses, don't install them. But there are likely tens of thousands of these in the US. And what has been discussed here are some reasons they are installed along with best practices if they are used.

 

[alchemon]

It might be semantic and I understand that, but we're engineers and should be speaking in technically correct language.

...

We say "pushing gas" because saying that a regulator "passes gas" makes some people giggle !!!

 

[Feedyourhead]

20 psi could make a huge difference. Imagine running your 200 psi system at 180, or your 100 psi system at 80. Imagine if the transmission pressure drops by 600 psi. 20 psi could save you.