Mysterious fires at regulating stations

[ICman]

We have had 3 small fires at distribution regulating stations, occuring near a control cabinet. The cabinet houses a "motorized regulator". We send a 120VAC signal from our RTU to a small motor, which turns the actuator screw on a Fairchild Model 10 or 64 regulator. The Fairchild regulator controls natural gas loading the main line regulators. This is how the pressure outlet setpoint is manipulated.

We've used this special setup for 30 years, and have never had a problem. Now, these fires occurred in the last 6 months.

We have limit switches on the motor to avoid pressure extremes (they switch power to the motor and are not monitored). The motor and switches are not sealed.

We have noticed excess gas venting at some sites. Others are venting gas thru the adjustment screw, into the cabinet. We believe this is the source. Ignition is probably caused by the arc in the switches.

I'm just wondering if anybody else has experienced this? If so, were you using Fairchild regulators?

How do you control the setpoint of your main distribution station regulators? I know we use an antiquated method, but it works and it's CHEAP!

Any other thoughts or opinions?

 

[CostasV]

Just one thought ...

If I understanf correct, gas is venting in a closed cabinet. Can you direct the venting gas through a pipe to a high (about 3 to 4 meters above ground) position in open air?

And a question. Were these fires actually seen by someone, I mean were these fires really been "feeding" with gas, or were they just combustions of the mixture gas+air inside the cabinet?

 

[ICman]

We thought about venting the cabinet, but there is no guarantee that it would be free of natural gas. I think that would only be a band-aid solution. We have tapped the vent on the regulator at some sites, and tubed it away.

Unfortunately, the fires were discovered after damage was done, so there isn't much to go on. We are examining one cabinet, but it's hard to tell if original ignition was inside or outside.

My Emerson Process (Fisher) guy tells me that other gas companies have experienced this. Of course, they now use Fisher's Proportionaire (?) product, which solved the problem.

Thanks for the reply.

. . . Steve

 

[McMoe]

Another question I have before I can give a good answer Is this a demand station or do you actually flow control or pressure control from a remote site. Second question is your gas and electric interface set up for Class 1 Division 1 electical connections. If this is just a demand station there are many combinations of regulators to hold set point.

 

[ICman]

We control from a remote site.

Limit switches and motor are NOT classified. I think this is a problem, as this seems to be a Div 2 area.

We are looking at IS type control. However, most people here are very reluctant to try anything new.

 

[McMoe]

If you absolutely must pressure control from a remote site and can pressure control can not be performed pneumatically there is a device manufactured by Becker Precision equipment in Elk Grove, IL that we have had good success in combination with Ball valve regulators. Not knowing your specific pressure and flow conditions it is hard to help with the controls.

Are you running this from a Gas Control operation or in a plant? Your term was distribution which can mean many things.

Be glad to help any other way.

 

[ICman]

I might be interested in the device from Becker. We are currently looking at a ProportionAire instrument designed for this use, but modified for our specific requirements.

I work for TXU in Dallas, TX. We are involved with natural gas transmission and distribution. The distribution areas include businesses and residential, with pressures regulated to well under 100 PSIG. A common setpoint is 30 PSIG.

Setpoints are remotely set from a master controller thru either Bristol or DGH telemetry equipment.

I am fairly new to distribution, working in transmission prior to this. Transmission uses the same philosophy, but with much more sophisticated equipment and higher pressures (600 PSIG +).

Thanks for the reply.

 

[McMoe]

Unless you have very high limits on managing your daily supply takes from your pipeline suppliers you would be much better off just managing the set points using pneumatic downstream control. Most regulators such as Fisher or Grove or even Rockwell provide simple solutions using spring or diaphram regulation and pilots to establish the set points. E& N Engineering in Westmont or Darien Illinois can help you with this solution.

Since you are working with residential and commercial customers you really can't control their consumption. The company I work for manages most of these systems using demand control, which means the operating personnel set the regulators to maintain a set downstream pressure. If we only want certain regulating system to feed or supply a system we may set the regulator back to a lower pressure. This way the regulator is satisfied and does not feed or allow flow. We, as most distribution companies, have operated this way for decades.

Transmission systems are a little different in that they operate under flow control. In these cases an RTU is used to perform flow calculations on sight based on differential pressures. The required flow is set and the regulator moves to maintain the flow rate. Of course in these cases the pressure varies; whereas in the example above the flow varies.

You can operate a system using both methods in tandem depending upon the supply problem you are trying to solve for.

E&N has experienced people who have designed and operated these types of systems.

 

[ICman]

Our field technicians also set pressures on local regulators. Some of our loads are extremely variable. The temperatures often swing 50 or more degrees in a day or over a week. Sending people out daily to hundreds of stations is wasteful.

Distribution uses a master (low point) controller with remote setpoint from Gas Control. As demand increases, the pressure drops, so the master pressure regulator opens. When it is almost full open, the flow in the line must be increased by increasing the upstream pressure controller setpoints. Due to the length and number of stations, there may be a substantial pressure drop between the furthest upstream station and the master.

It's challenging to balance loads without exceeding maximum allowable pressure in the upstream stations. Therefore, Gas Control may frequently adjust pressure setpoints.

We've only been using the remote controls for 30 years. Before that, setpoints were manually set.

Have you seen the Fisher RegFlo regulator? It's a standard EZR model regulator with a built-in "RTU". You can set and monitor several parameters over the Internet. We have a couple of Beta units installed. Of course, this has nothing to do with fires, but I thought I'd throw it in .

Thanks for the comments.

 

[ColCol]

Hey IC Man,

You have a serious situation there. The electrical equipment at your regulator station is clearly not designed for the proper hazardous area classification.

That said, you may be igniting gas from switch contacts, but you may also be overheating some wiring, causing a fire without any gas fuel. Since the "designer" of the station screwed up the hazard classification, he may have also messed up the overcurrent protection, making a wiring fire quite likely.

I've used your Jordan/Fairchild motors before many times, they work and are very reliable. You do need electrical engineering help, however.

 

[jsummerfield]

The gas venting within an enclosed space is a serious hazard. This likely requires a Class I Division 1 rating for the limit switches and other accessories as the venting is continuous, not the result of a malfunction.

Intrinsic Safety is costly and not recommended for unsophistocated users. All IS wiring must be segregated from non-IS and grounding must be within one-ohm of earth. None of the solutions are new, only new to those who do not know what they are doing.

It sounds as though you have been increasing your knowledge of the situation. Continue to consult your Fisher-Rosemount and other equipment providers. If you can provide some better description of the situation you might get some reasonably unbiased opinions here. Consider some contract engineering support if needed.


John

 

[ICman]

Thanks for the comments and suggestions. One fix that has been implemented is the addition of a seal on the adjustment screw. We are still looking into the ProportionAire device for use with new stations.

BTW, I have been working with Intrinsic Safety systems since '75. My company uses IS at some sites (grounding is a BIG issue, with or without IS), but it is not common in gas distribution.

The correct implementation of IS can be expensive, sometimes more so than explosion-proof enclosures. Sometimes the benefits outweigh the costs (easier maintenance, lower cost installation, etc.), but not in minor applications. I think the replacement of unclassified equipment in this application would vastly improve the situation - I just gotta talk others into it .

 

[TTheHammer]

I hate to bring up a dead topic alive again, but I couldn't help it.
anyway, the problem I think is 2 parts, the first one is gas escaping from one of the control elements which is either the valve or the swtich, and the other part is the case of fire, which is mainly beacuse installation was not done according to the NEC 500 (NFPA 70), as either it would need IS installatiuon or Class I ,Div 1, group D installation (i.e explosion proof fitting), as the gas vented here is under normal operating conditions.

Thanks for all the replies, I just thought to add this for later reference.