5 replies to this topic

[xavio]

Dear all,

 

When checking the recip compressor PID of one vendor I realize that they want to install restriction orifices (RO) on suction nozzle and gas discharge nozzle of the separator vessel.

The compressor is 3 stage, W type, 0.15barg to 15barg, 250kW.

I have done some study and found out that it is quite common for some vendors to add RO in order to suppress gas pulsation/vibration; it is in line with what the vendor of this particular compressor says.

 

The problem is I have never seen such design employed in our existing recip compressors.

Other vendors competing for this project also do not use the RO.

Some articles I've read indicate that the RO is an effective, cheap way to suppress pulsation problem for small recip compressor where detailed pulsation analysis is usually not performed.

However, as Mr. Montemayor pointed out here:

http://www.eng-tips.....cfm?qid=126517

 

"5.  In my experience, I’ve never seen any success of pulsation dampening by the use of restriction orifices.  To me, this is all a waste of precious HP energy.  If your discharge piping is of a sufficient size and it is installed with a minimum of turns and convolutions, valves, fittings, and has a discharge snubber vessel with a liberal capacitance volume, you should have no problems."

 

I am not sure about RO's usefulness, and concerned that it is really a waste of energy.

 

Upon more detailed comparision, I realize that this particular vendor chooses to eliminate discharge volume bottle right after compressor discharge. Instead, it combines the functions of discharge pulsation dampening, suction pulsation dampening, and gas/liquid separation into one "separator vessel", which is located downstream of intercooler.

I am inclined to think that this particular vendor wants to reduce manufacturing cost by substituting discharge volume bottle with RO. Any increase in energy consumption will be us owner to bear.

 

Questions:

1. Is the RO really necessary and effective?

2. Can someone share his/her experience in using W type compressor, compared with vertical or horizontally opposed ones?

 

Thank you so much.

 

xavio

[Bobby Strain]

I would ask your vendor for reference installations. If this doesn't satisfy you, don't buy his equipment.

 

Bobby

[xavio]

Hi Bobby,

 

This vendor has offered a very competitive price, we don't want to strike them out if we're not sure.

Although not so sure with the plus and minus of it, I think this is a quite common practice by some vendors, you can find literatures on this issue.

Do you have any experience with the RO yourself?

 

Thanks.

 

xavio

[Art Montemayor]

Xavio:

 

I concur with Bobby Strain’s recommendation - except that I make a distinction between a vendor and a manufacturer.  I would recommend going to the manufacturer and not the vendor if the vendor is simply a “packager” of compressor systems.  I make this distinction because unlike many years ago, the manufacturer –especially here in the USA - is no longer the seller of the compressor system.  Manufacturers now supply their compressors to “packagers”, fabricating and design companies that design and bid on the packaged compressor system.  The packager (vendor) decides what auxiliary equipment is employed (and how) on the compressor system.  The manufacturer is not normally aware of what is bid on the proposal you submit to the vendor.  Therefore, if you want to ensure that the compressor is safely protected, the best thing to do is check with the manufacturer to make sure that the vendor is following prescribed best practices for the machine.

 

You are, in my opinion, correct in challenging your vendors.  A packager is competing on price with other vendors and it is only natural to expect that the vendor should try to maximize his chance of supply with a low price while also protecting his profit margin.  However, it all reverts back to you, as the buyer, being in control and insisting on your specifications and requirements being met satisfactorily.  You will have to live with and maintain whatever you accept as the supply.

 

You say you have a “W” type of reciprocating compressor.  At 250 kW, this is a rather large machine.  The largest “W” type I have operated has been a 125 Hp ammonia machine.  At 250 kW, your machine is probably direct-coupled to an electric motor and consequently is running at the asynchronous speed of that motor – a relatively fast rpm for a recip.  Am I correct in this assumption?  If so, then your pistons are all trunk type pistons with compression only on the head end.  This type of arrangement is used in relatively compact, skid-mounted systems that do not rely on a concrete, monolith poured foundation.  As a result, you have to be mindful of sustained mechanical vibrations and created pulsations because these can cause a lot of damage and harm in such a system.  Because of the skid-mounted design, space and foot print are a premium and packagers are prone to rely on the minimum amount of equipment that they can supply.  Consequently, the use of volume bottles demands a larger-than-normal skid arrangement and a higher fabrication cost.  The buyer should be aware of this kind of typical engineering tradeoff when specifying the compressor package.

 

To answer your specific questions:

1. The necessary use of a resistance orifice depends on the specific application and in this case, it could be based on the need to conserve space.  However, this doesn’t mean that the RO would necessarily meet API 618 specifications for pulsation mitigation.  The necessity for the RO could be substantiated by the bidder guaranteeing the pulsation results to your satisfaction – something I doubt a bidder would be willing to guarantee.
2. I have operated horizontal, vertical, and W type of compressor arrangements.  The W’s that I operated were all belt-driven, which means that they were running at relatively slower speeds.  All the compressors I operated were equipped with volume bottles (snubbers, dampeners, KO pots, etc.) to reduce the related pulsations.  I always used conservative pipe sizes and very short runs with minimum turns.

 

 

[xavio]

Dear Mr. Montemayor,

 

Thank you very much for sharing your valuable experience.

I never knew that such "packagers" even exist, I've never met them I suppose.

As long as I know, the vendors I'm dealing with are all compressor manufacturers, European companies.

 

The compressor's pistons are double-acting crosshead type pistons, they run at 500rpm.

I read that trunk type pistons are only used for <100kW, I don't have any experience with them.

 

You're right, it is a direct-coupled to an induction motor, but the motor is 14 pole.

Originally one vendor proposed V-belt, it said that V-belt could be used for up to 300kW machine (references given).

Because our mech. engineers resent using V-belt, we have forced all vendors to employ direct coupling.

Besides, API 618 do not recommend V-belt beyond 150kW machine.

May I ask your opinion about this 150kW limit?

 

You're also right in saying that we have limited space, and budget as well. ()!!!

I think conservation of space is also the reason why the vendor proposes the RO in place of volume bottle.

 

After reading your suggestion, I think I will approve the RO design, it is part of vendor's know-how and responsibility.

The RO, after all, is not something unacceptable or illegal.

Anyway, the vendor, and all other vendors, will not guarantee any vibration value.

So, we should see this as owner's risk for choosing the more economical option.

 

Lastly, do you agree with the opinion that W type is the cheap version of vertical type, so it is generally less reliable?

How is your experience with this type of recip compressor?

 

Thank you once again.

 

xavio

[Art Montemayor]

Xavio:

 

My opinion of the “W” type of reciprocating compressor is that it is a manufacturer’s response to the need to be more economically “competitive” and not necessarily to comply with a user’s need for robust, reliable, easily maintainable, and low-wear features.  As a result, what one gets is a high rpm machine that has a cylinder orientation that is difficult to access or inspect while running and valve locations that make access difficult without elevated platforms and ladders.  The piping around the cylinders and associated intercoolers, snubbers, and separators requires a larger foot print than anticipated.  All these features come as a result of specified compactness and forces the user to adopt to the inconvenience and difficulties or modify the installation for needed operational room and accessibility.  In the end, the final product comes out requiring more space, room and structure than was anticipated – inflating the final installed cost.

 

Working on the crankshaft bearings, connecting rods, and crossheads of machines that are other than horizontal can be very taxing, laborious, and cumbersome.  Removing the pistons and associated rings is even more taxing and requires special tooling.  I’ve only operated refrigeration “W” machines, the biggest of which were 125 hp.  The “W” type is one that is definitely not on the “wish” list of Operations and Maintenance engineers.

 

The high rpm and basic, competitive features of these type of compressors do not favor a long operational life when compared to the traditional, slower (and more expensive) horizontal, balance-opposed machines.  You don’t see too many “W” types that are 30 to 50 years old and still operating.  Whereas, with horizontal balance-opposed machines you can still see many old models still in operation.

 

As in everything else, you get what you pay for.