reciprocating gas compressor unloading 3

[Echo82]

Good Day

I'm working in an engineering company and one of our projects is a gas compressor station which includes 4 trains of reciprocating gas compressor. Currently we are in detailed engineering phase of that plant. The issue that we have in most of our projects (which are mainly gas gathering and compression plants) is that the process of the plan requires compressors to work under 25% or 50% of their capacity for long period of time( in some points during life cycle of compressors) which is very harmful for compressors and valves and causes excess heat in compressor body, valve and... . the main method that we use for unloading is suction valve unloader + spill-back( cooled bypass from discharge after-coolers to compressor's suction scrubber), but we prefer valve unloaders.
someone said to me that a hoerbiger HydroCOM system has overcome the the issue and we wont have overheating of our compressor if we install that system and it's the only solution if we want to operate compressors under 25% or 50% unloading continual.
but I don't understand how a valve unloading system(even HydroCOM) can solve that problem. any one have similar experience? or is there any way to perform continual unloading without any problem?

Thanks in advance.

 

[BigInch]

Try more than one compressor.
Seems like a better way than forcing dependence on one vendor's system.

What would you be doing, if you knew that you could not fail? Ans. Bonds and derivative brokering.

 

[Echo82]

Each Train consists of 3 reciprocating compressors (2 working+1 standby) and final train compress the gas to about 2000psi. So considering the capacity of compressors (each about 15000 kg/hr depending to the stages), increasing the number of compressors doesn't seem economic, but it is practical I guess.

 

[DLite30]

The semi-active hydrocom system won't help much down at those low of capacities.

Firstly, you need to determine what the minimum load the driver needs to maintain...for example, a Caterillar gas-fueled engine you probably want to keep it loaded to at around 60 to 70% load minimum....they advertise 50% load, but I'd be conservative. So, that's one contrainst, you'll have to consider, along with minimum speed, discharge temperature and gas cooling capacity.

You mentioned pockets, valve unloaders and recycling (spill-back), all of which are good capacity and load control options, as well as speed control. Another option, is suction pressure control. You cand affect both compressor/engine load and throughput by lowering your suction pressure, or even discharge pressure.

 

[Echo82]

Drivers are electric motors and speed control is possible. But because of vibration and resonance problems of variable speed drivers in different operating points of compressors we are affraid of them and prefer to use VFDs for centrifugal compressors. Reducing the suction pressure is applicable but since we have four trains in series( four stage of compression) I think it affect the perfomance of each compressor and final stage discharge pressure. Afterall I think choosing a variable speed motor and performing a full dynamic study for all possible speeds and capacities might be better way comparing to unloaders and recycling.

 

[bcs5274]

are these units in series (one feeding the other) to achieve higher pressures/flow, if so would be more practical to utilize a 6 throw frame in four or five stage configuration and have 2 operate and 1 standby or more , just a thought/question

 

[JJPellin]

We have groups of Hydrogen compressors that run at 50 to 75% load for years at a time. The main group that I am most familiar with consists of two, four-cylinder, three-stage, 4000 HP, Dresser Rand compressors (HHE) which operate in parallel. Much of the time, they only need a little bit more capacity than one compressor would provide. But, rather than starting and stopping a machine this large, they run both compressors all the time and vary the loading between 50% and 75% using suction valve unloaders and spill-backs. We have had excellent reliability from these machines running this way. Since we redesigned the valves, unloaders, high pressure packing and lubricators, we routinely run these compressors for more than 2 years between maintenance shut-downs. We have continuous monitoring for valve temperatures, gas temperatures, frame vibration, rod loading and rod-drop. This system is armed to trip the compressors at appropriate limits. I am not familiar with the HydroCOM system. But, I don’t see how it could be much of an improvement over our existing system. I have never considered continuous operation at partial load to be a problem.

Johnny Pellin

 

[Echo82]

bcs5274

Yes, they are in series and each compression stage(train) feeds the other one till it compress the gas to 2000psi. I think a 4 throw frame would be enough for our purpose, because it's hard to achieve 25-50-75% unloading on a 6 throw frame if we going to use the valve unloaders only.

 

[bcs5274]

on the four throw frame you could still utilize a five stage configuration by running a steepled cylinder for 4th and 5th stage

 

[dcasto]

let me get this straigh, 3 compressors in parallel by those three 4 times in series? that is 12 compressors with 4 in spare mode?

That doesn't even make sense to me. The logic and controls will cost more than the units.

put in 12 4 stage units, then use unloaders spillbacks whatever and you'll get a low end of 3% of design flow if you need to be that low.

 

[BNiessing]

I know I am late to this party but what speed range are you looking to operate at? HydroCOMs or any type of stepless capacity control device installed on suction valves are very low on my list of capacity control methods I would installed on high speed compressors (900 RPM or above).