2 replies to this topic

[iceman53y]

<br>

Attached Files

•  NGC Datasheet.pdf   131.43KB   66 downloads
•  NGC1DVCE Cyl A 003.jpg   184.5KB   13 downloads
•  NGC1DVCE Cyl A 006.jpg   237.77KB   13 downloads
•  NGC1DVCE Cyl A 008.jpg   1.83MB   9 downloads

Edited by iceman53y, 16 January 2011 - 11:59 PM.

[Art Montemayor]

iceman53y:

You give some detailed information and good background information; however, we still lack some vital information:

• What gas are you compressing? Is it natural gas? Is it clean?
• Are the compressors operating 24/7 (24 hrs per day and 7 days a week)?
• Were the valve problems happening originally with the 2-stage configuration? In which stage(s)?
• Have the valve problems persisted after the addition of the 3rd Stage? If so, in which stage(s)?
• Who originally specified this compressor type and supplier?
• Who and how was the decision made to purchase this type of High-speed reciprocating compressor (API 11P) over the more rugged and robust API 618 type?
• Were the valves specified by the client or by the supplier?
• Is the compressor performance still under warranty?
• Are the condtions in the data sheet the ACTUAL existing process conditions, or those specified by GE?

My response to your specific questions is:

The possible causes of this kind of failures can be divided into DESIGN problems and OPERATIONAL problems. Design has to do with high valve lift, excessive valve impact velocities, compressor speed, cylinder pressure, and valve springs. Operational factors can be solid particles in the suction or created within the cylinder and piston wear.

This type of failure is VERY common – BUT ONLY IN badly specified or applied compressor designs and operations. ALL reciprocating compressors can be made to operate trouble-free. HOWEVER, if they are wrongly applied in an application, troubles will soon follow. The design and specification engineer must apply the correct type of reciprocating compressor for the application, taking into consideration the PROJECT SCOPE OF WORK (SOW). If the SOW calls for a high-speed, fast-wearing, rapid depreciation type of machine is called for, then everyone should be made aware of the inherent trade-offs that follow: high maintenance, rapid wear, and high down-time.

The steps and measures you can take to find out the root cause of the problem depends on a full exchange of information between your valve manufacturer and your operations. You must be willing to provide complete operational data and the valve manufacturer must be willing to review and modify his valve design. The circumstances surrounding each specific case of valve failure and its resolution are unique.

There is a time and place for each type and configuration of reciprocating compressor. For example, if you are applying a recip out in an isolated, remote location where it will compress field natural gas from well sites that have a limited life expectancy, then you probably are prudent if you consider applying a high-speed type compressor for that application.

However, if you have a base-loaded, gas compression station that is feeding an important infrastructure on a 24/7 basis then you certainly should be applying the API 618 standard type of machine – which is a more robust, heavy-duty, and slower type of machine. If you try to apply an API 11P type of machine in this case because you want to save capital monies, then you should be prepared to face the consequences after a while.

You basically have inherited a very serious problem. Your initial design was a failure. This is not an attempt to insult or downgrade your original design; the fact is very clear: your original compression ratios were ridiculously high and the machines should have had 3 compression stages (with efficient intercooling) from the very outset. Now, the machine has been “modified” with a more conservative outlook. But you basically still have the running conditions of a cheaper, high-speed machine. You do not own an API 618 machine – no matter what your GE representative says or what you prefer to believe. You purchased and installed an API 11P type of machine that is more “price competitive” than the API 618. You basically cannot expect 618 performance from an 11P design.

I recommend that you work directly with GE to resolve this valve problem – only if the valves were designed and furnished by GE. I suspect that GE may have sub-contracted the valve design and supply and should that be the case, you either work directly with the valve manufacturer or seek another world-class, recognized valve supplier to study and resolve your valve problem.

There are several different types of compressor valves, each of which has application limits, so a good
understanding of how to reliably apply each type is paramount. A successful resolution of valve failures often means having to employ new or redesigned valves that may be less efficient. Therefore, sacrificing some compressor efficiency for increased valve durability may wind up as compromise. The timely resolution of valve failures depends on accurately analyzing complete operational data and any available failed hardware, so any study of your problem will involve some field work.

I have made this a lengthy response because I consider your valve problem as a very serious and important problem to resolve. From the size and type of compressors you have installed, there is a large amount of invested capital money sitting there and not being used efficiently in operation. This must be resolved – and as quickly as possible.

I hope this personal experience is of help to you.

[Art Montemayor]

Iceman:

I strongly recommend you install a coalescing filter in the suction of the 1st stage of each compressor. This is recommended as long as you can accept the pressure drop with such a filter.

Your first stage seems to be the most affected – as reported by you. Therefore, gas impurities may be a problem. However, without a rigorous valve analysis and design (in view of the need to add another compression stage) it is difficult to assume that you have the required and applicable valve types and designs. I would seek the help of recognized valve expert manufacturers if your original compressor “packager” is now in the background and is not interested in resolving the problem created by him.

As stated before, you basically have a high-speed machine and one that normally is susceptible to valve problems due to high valve velocities – unless specifically designed for the application. I am not opposed to the use of API 11P type of compressors as long as we all know the inherent trade offs that are common for this type of machine. A cost-effective machine has its limitations and since you are obviously encharged with operating these type of compressors, it is wise if you become very familiar with all the trade offs involved so that you do not waste your time and money trying to make these machines into another type. The basic design can be modified – but it will cost capacity and money.

I am attaching some material for you to study in case you are now getting familiar with your machines.

Answers to your further questions are:

1. The only problems involved in running these compressors on recycle mode (using your recycle valve to send discharge gas back to the 1st stage suction) for longer periods of time is that you lose capacity. I believe that is fairly obvious. Of course, this is also assuming that you are cooling the recycle gas before introducing it into the 1st stage suction. You also are wasting all the energy input that you have used to compress the gas. I don’t know what gas capacity you require, but this type of operation is very energy wasteful and I personally cannot recommend this as an answer to your problems. The answer to the basic problem is to fix the valve problems. Your valves should be lasting between 3 to 6 months of operation – running 24/7. You basically have a very serious problem and should work to resolve the CAUSE – not treat the EFFECTS.

2. Please refrain from using acronyms without first defining them. I believe that a “VVCP” is a Variable Volume Clearance Pocket. If so, then refer to the tutorial material I have furnished you and understand that when you INCREASE the clearance volume in a reciprocating compressor, you are DECREASING its ability to compress the design quantity of gas it was design for. In other words, you are compressing less gas per unit of time. You didn’t furnish us a drawing of your clearance pockets, so I don’t know how they operate. Normally, when you RETRACT THE piston of a variable clearance pocket, you are increasing the clearance volume. Therefore, you would be decreasing the compression capacity. Depending on how much you open the clearance pockets and on what cylinders, you will affect the compression ratio. Your GE compressor operating manual should indicate this clearly to you. You should be in contact with GE or your packager, whichever is responsible for the performance.

I would pursue the issue of proper and acceptable valve operations because the high-speed reciprocating compressors normally require a special valve design. This is just common sense, since the valves are operating much more than they normally do at the lower speeds (rpms of the crank). This is just one of the inherent trade offs of an API 11P type of machine (what you have). An API 618 type of reciprocating compressor has much lower crank speeds and less piston speeds as well. This makes the valves work less and therefore the wear and tear is also much less.

Attached Files

•  Reciprocating Compressor Capacity Control.xlsx   214.47KB   80 downloads
•  Recip Compressor Valves & Unloaders.doc   449.5KB   59 downloads