Dry gas seal protection of hydrocarbon pumps and compressors 2

[ENG7345]

I would be grateful if you could assist me with a short response to the following queries:
• Is automatic mechanical seal failure trip (either primary or secondary seal) implemented on pumps that are handling hydrocarbons of C5 or lighter fractions?
• Is it normal practice to monitor the health of both the primary and secondary seals on a continual basis?
• If the trip is not automatically initiated what is the operations response philosophy? E.g. alarm only, operator attends and manually shutdown pump.
• If a trip is initiated does it only stop the pump or are the isolation valves closed as well?
• On compressor dry gas seal systems is a trip automatically initiated on primary seal failure?
• On compressor dry gas seal systems is a trip automatically initiated on secondary seal failure?
• On compressor dry gas seal systems is a trip initiated on failure of the separation seal?
• Does the response to the queries above differ between onshore and offshore environments?

Thanks

 

[LittleInch]

This is a multi discipline answer you need from process, tech safety, instruments and controls and the mechanical engineer.

"Failure" needs to be defined. All seals will "leak" to a certain extent - the vendor will need to give you acceptable flow ranges which you then monitor and alarm at high, but keep running until you can change over and repair. If you get to HH then trip.

If you have double seal systems then whether you trip on primary or not is up to you to decide as part of your operating and control philosophy. If you trip on primary failure, then you lose flow if you don't have a spare. For me a secondary seal gives you time to switch or ramp down without the shock to the system of a sudden trip. But monitoring is the key to be able to see if the seals are wearing and about to "fail".

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

 

[ENG7345]

Hi LittleInch,

Thanks for your reply. About allowable amount of leakage you are right. You are also correct about trip at high-high leakage. My question is whether this trip should be automatic (hardwired) or manual (by operator)?
For both dry gas seals of the compressors and mechanical seals of the pumps we use tandem seals.
Currently we only monitor leakage on the seals and on high-high alarm, operator shuts down the machine manually. I am wondering if it is the common practice across the industry. Please note that I am interested in Natural gas compression and light hydrocarbon pumping.

Thanks again.

 

[georgeverghese]

Seal failures on compressors and pumps in flammable fluid service, when these do not result in an auto stop and full auto isolation of the unit, can escalate into major incidents vary quickly. A manual stop / isolation is a poor choice in these services. It would be better to bite the bullet on schedule and cost implications in the engineering phase ( if that is what this will turn out to be) than to handover to plant operations an unsafe to operate pump or compressor.

 

[rotw]

I hesitated to answer to this because getting such a bunch of question (listed and not all related one another) does not look like we are having a technical discussion on the point. No offense intended, but to me - it looks like we are doing OP's "homework".

 

[LittleInch]

ENG7345

This goes to the heart of your safety system philosophy and design.

In general for alarms and trips, the sequence is:

Normal range - monitored by operators / controllers.
Variable set point warning - some systems allow the controllers to set a warning that some variable is approaching a high limit
High level alarm - An alarm which needs to be accepted and is recorded in the control system log. The intent of an alrm is for the operator / controller to do something about it. What that is will vary so he could shut it down, start the spare first then shut it down or within a set period of time adjust something else and reduce the level to below the alarm level. what should not happen and it sounds like that at your plant is basically ignore it and hope it goes away or just leave it. That is bad practice.
High high level - the unit is automatically "tripped", shut down, made safe, isolated by the Safety system. Relying on a human to do this is very dangerous.

If you ever read accounts of incidents the human factor in both interfering and overriding things and also getting overwhelmed with alarms happens time and time again.

Most designs now look at the number of alarms generated and whether the operator / controller can actually do anything to prevent it reaching high high and tripping. For a seal gas system they can, but the correct action on ahigh alarm is to shutdown the machine after starting your spare and then fix the seal.

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

 

[ENG7345]

Thanks so much for your recommendations.
Is there any standard for alarms and trips of mechanical seals failure?
I found some guidelines in API-614 for dry gas seal systems (see attachment), but nothing for mechanical seals.

 

[rQuestionEngineering]

• On compressor dry gas seal systems is a trip automatically initiated on primary seal failure?
yes
• On compressor dry gas seal systems is a trip automatically initiated on secondary seal failure?
yes
• On compressor dry gas seal systems is a trip initiated on failure of the separation seal?
no
• Does the response to the queries above differ between onshore and offshore environments?
no