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Rotating Equipment Sparing Philosophy


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#1

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Posted 18 February 2013 - 09:56 AM

Is there a philosophy / guideline for sparing rotating equipment ?

How does one decide if one needs to provide a stand-by ?

If possible, share guidelines of licensors.



#2 thorium90

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Posted 18 February 2013 - 09:58 AM

Reliability issues in a continuous plant operation process. In order to allow the process to continue running in the event an equipment fails or requires maintenance, one spare is used. If it is critical enough and its downtime will cause enough financial loss, then a spare is usually bought.


Edited by thorium90, 18 February 2013 - 10:00 AM.


#3 kkala

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Posted 18 February 2013 - 05:37 PM

Link <http://www.cheresour...ty-calculation/> can be useful, but mainly as a theoretical background. From what I have seen, you seldom decide to install additional standby equipment on an existing unit. The decision for standby equipment is mainly made during design of the plant, according to the experience of specialized detail engineering contractor and even of licensor.

Suppliers have a knowledge on their operational availability (OA) of their equipment, yet the designer specifies standby equipment (installed spares), often demanding specific operational availability.

Experience plays a vital role on this, but I think theory also helps in combining operational availability of the plant with that of each individual piece of equipment. For instance, suppose that you need 64500 ton/y of a product made in a plant of 92% OA. A piece of equipment would have maximum normal capacity c  corresponding to 64500/(0.92×365×24)= 8 ton/h of product (briefly expressed as c=8 ton/h here below).

The plant needs a pump of c= 8 ton/h, but its OA=90%. This would make plant OA <90%. Same pump can be additionally installed as standby, so that  probability of one pump operating is 99%. Another choice is to install 3 pumps×c=4 ton/h, two operating-one standby. Probability of two pumps operating = 97.2%. See above mentioned link for the estimation of such combined probabilities.

If this combined probability is lower than 92%, matter needs further investigation. If higher than 92% experience affects the decision, since the pump may get out of service unexpectedly, blocking plant operation. The more  pump dead time (10% of the total) can be covered by planned maintenance, the closer to 92% this combined probability can be. Whole situation is hard to be modeled, experience completes it.

Heavy expensive equipment pieces (e.g. fans, heavy pumps) have no standby and plant stops for their overhaul. Their OA shall be sufficiently higher than the plant OA and  their planned maintenance should be maximized. But the principle of OA is not applicable for a small part of critical equipment concerning safety, e.g. fire water pumps.

It is noted that if above plant OA were reduced to (say) 85% (c=8.66 ton/h), standby equipment would decrease. On the contrary standby equipment would increase significantly if OA approached 100% (c=7.4 ton/h), which was applied in a local Alumina Plant design (1989). For petroleum units OA is around 92%.

I suppose above theoretical base is also written in books, but above is only result of discussions during project implementations.

Some seen examples of standby equipment, not necessarily applied as is for all kinds of plants:

BFW pumps: 1×100% operating, 1×100% hot standby (autostart)

Boiler fuel pumps:1×100% operating, 1×100% hot standby (autostart)

Fans: no standby in general

Firewater pumps: 2×50% operating, 1×50% autostart stanby (2 of them diesel driven)

Phosphate grinding mill: no standby

Phosphoric acid filter (dihydrate): 1x100% pump, standby for any of ~5 pumps operating.


Edited by kkala, 18 February 2013 - 05:45 PM.


#4 breizh

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Posted 18 February 2013 - 06:58 PM

Equipment per equipment you need to perform the analysis and evaluate the consequence of not having spare ( in the workshop or in the storeroom) . This should be done with the support of the maintenance team and manufacturing who are well aware of the risks ( safety , quality, operation ,....).

 

Good luck

Breizh



#5 kkala

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Posted 19 February 2013 - 04:21 AM

I also remember a case of two reciprocating air compressors (2×100%) in a plant basic design, one operating - the other standby. Later in the design phase they were replaced by one centrifugal compressor (1×100%). The centrifugal compressor is considered more reliable than reciprocating (one to one), that is of higher operational availability (OA). The latter is also called operating factor here, specific for each piece of equipment. But indeed (post no 3) all cannot be expressed through mathematics, experience plays a vital role.






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