How to fix PAHH, PALL, LAHH, LALL etc set points 3

[DoraeS]

Hi, everyone
I would like to ask about when designing the plant or platform, how do we fix the set points for high high/ low low level/pressure/temperature/flowrate shutdown? what is the relationship with the normal operating point?
Is there any API codes for my reference?

Thanks for everyone's great help

 

[Montemayor]

llc0612:

First off, I don't believe there is any API recommended practice or "code" to go by or go to. I have used the engineering criteria pertinent to the process. In other words, you require thorough knowledge of the process and it's capabilities and, knowing these, you apply logical and safe criteria to each specific alarm or shutdown activation.

For example, the LAHH (Level Alarm, High-High) is usually the "last resort" prior to an emergency shutdown of the unit in question. Taking this into consideration, I always try to allow for sufficient, safe operational time to take the necessary steps to avoid the ultimate shutdown and employ an alternative plan. This involves expert instrumentation knowledge of the speed and response involved with the instruments being used as well as the maximum flowrates of fluid entering the subject vessel. Other details may enter into the logic employed - such as vessel location and type of vessel and fluids involved.

In my process design I always insist on a detailed and specific explanation of the set points for each alarm and shutdown point. These are reviewed and scrutinized in detail at the preliminary design, the final design, and certainly at the HazOp phase. I am accustomed to running at least 2 or 3 HazOps prior to the actual commissioning, so these points are discussed redundently.

In my opinion this is a very good and important question that often is not given the priority and serious attention that it deserves. I personally feel that all involved engineers should press and demand clear and accurate explanations of these functions from the responsible process engineer(s) - not from the instrumentation engineers, who unfortunately are usually kept in the dark regarding process characteristics.

I am interested in reading what other forum members have to say about this important subject.

Art Montemayor
Spring, TX

 

[Guidoo]

Again, a very valuable post from Art Montemayor.

In general there are no international standards that define setpoints for instrumented safeguarding trips. This is exactly the type of questions that require good engineering judgement (and that's what we get paid for, not for simply applying rules from standards).

Art already gave you some considerations. For example, a high pressure trip should be set high enough to prevent nuisance trips, but low enough to prevent opening of safety valves etc. Safety valves normally have a set pressure equal to the design pressure.

A high level trip on a compressor suction drum is set at a level above which you would get excessive entrainment. A low level trip is set at the lowest possible point on a vessel but still high enough to prevent vapour breakthrough.

It is not possible to describe every possible situation in this forum. Experience and detailed knowledge of the process ,the hazards the instrumented safeguarding is protecting against as well as of process dynamics is required.

 

[jsummerfield]

I think that API RP-14C (offshore safety shutdown) used to have a guideline. First define the normal operating range. I think that the high shutdown value was 15% above the normal range. Low may not even require a shutdown unless gas blowby would cause overpressure problems in the drain system. I do not have a copy of API RP-14C handy.

John

 

[cawse001]

Guidoo raised a good point about nuisance alarms.

The issue of "alarm flooding" is topical, particular it being recognised as a contributing factor in accidents, where the control room operator is either de-sensitised to alarms or overwhelmed during a process upset.

In assessing the requirement for alarms you may also wish to consider monitoring (and alarming) the rate of change of a process variable, in addition, or as an alternative to, having specific alarm points such as LAHH.

I would also echo Montemayor's point about recording the basis for alarms, particularly in the context of managing change during operations. I have seen a process plant incorrectly constrained because the high level alarm on a vessel was considered sacred and nobody knew better.

Cheers

 

[TrevorP]

The question to consider is - What are you expecting the operator to do in response to the alarm? If nothing, then is there really a need for it? If there is, then make sure it's set as a low priority, and that it doesn't continually go off during normal operation. They can be useful for troubleshooting etc., but there's nothing more annoying than a continually sounding alarm. Your operators will not only ignore it, they will also, over time, learn to ignore all the alarms. If one alarm goes off a year, you can bet an operator will respond it. If one goes off a minute, you can bet he won't respond to any.

If you are expecting the operator to respond to it, make sure that he has sufficient time to do so. Many major companies state in their internal guidelines that this should be 1 hour. What are the consequences of the operator failing to respond? Is there an automatic trip system? Is it clear what action you expect the operator to take? Has he received sufficient training? Quite often in an emergency situation, one upset will lead to another, and so the operator may have a dozen alarms going off all at the same time, and he/she may not have a clue as to what action he should be taking, even though it may seem obvious to you as the designer.

The only standards I know of are for critical instrument design. Refer to ANSI/ISA-84.01-1996 Application of Safety Instrumented Systems for the Process Industries (Available at

http://www.isa.org/Template.cfm?Sec.../Ecommerce/ProductDisplay.cfm&ProductID=2556)

in the USA, and IEC 61508/61511 (

http://www.iec.ch/)

elsewhere.

 

[jay165]

API RP 14 C is the required document for any oil production facility operating offshore (in US waters) and federal on shore leases. You (the operator) are required to submit Safety Flow Diagrams as well as a SAFE (Safety Analysis Function Evaluation) Chart. RP 14C has a detailed checklist for every process device common to oil production facilities and what action the protection devices must do - ie a high sump level shuts down the wellhead. To my knowledge, there is no equivalent document for refineries or chemical plants.

14C tells you why, what and where but unfortantely it does not tell you HOW to set shutdown or alarm levels. Like the other posts mention, this is based on reaction times and escalation factors and the ability to get the process back into control. The 15% mentioned earlier is not in 14C, it is an MMS requirement of what you must set your PSH's after 6 months of steady state operation.

 

[controlnovice]

I don't know of any standards for alarms, but there are several good papers out there on alarm management. For example, they recommend prioritizing your alarms: 5% High Priority (I would call Critical), 15% Medium, and 80% Low. I think there was a paper in Control Magazine (

http://www.controlmagazine.com)

in August or September 2003 about alarm management you may be able to find.

One guideline is: British Engineering Equipment and Materials Users Assn (EEMUA) at

http://www.eemua.co.uk,

which has a guideline, "Alarm Systems, a Guide to Design, Management and Procurement, EEMUA Publication No. 191."

Others: British Health and Safety Executive (HSE) at

http://www.hse.gov.uk/pubns/chis6.pdf

for "Better Alarm Handling" or the Technical Measure Document, "Control Systems" at

http://www.hse.gov.uk/hid/land/comah/level3/5c9933f.htm.

ISA TR91.00.02 "Criticality Classification Guideline" offers guidance in defining types of systems.

The Abnormal Situation Management (ASM) consortium may have some good articles also.

http://www.asmconsortium.com

There are also several companies that perform alarm management studies that can help you. Matrikon Inc is one that comes to mind.

 

[SooCS]

MMS mandate basic office safety instrumentation setting requirements. Check it out under 30CFR. Can't remember which series of the CFR.

 

[SooCS]

oop, typo error. dont know why I type in "office" for "offshore"