7 replies to this topic

[shan]

Hi Everyone,

We are studying a three stage compressor (suction pressure: 45 psig and discharge pressure: 140 psig, 400 psig, and 1000 psig) blowdown flow rates and time. There is only one blowdown valve after the 3rd stage discharge drum, one shutdown valve at the upstream of the 1st stage suction scrubber, and no check valve between stages. If we want to determine the blowdown time to 100 psig, should we do 1000 psig to 400 psig for the 3rd stage discharge cooler and drum, then 400 psig to 140 psig for the 2nd and 3rd stage discharge volumes, then 140 psig to 100 psig for 1st, 2nd, and 3rd stage discharge volumes? Or do we do a blowdown calculation for 1000 psig to 100 psig for all the volumes including the 1st stage suction volume (45 psig)? Or do we do a blowdown calculation for an equalized pressure (between 1000 psig and 100 psig) to 100 psig for all volumes?

Regards

Shan

[Art Montemayor]

Shan:

Please be specific when furnishing Basic Data. You have failed to tell us the most important data: is it a positive displacement or a dynamic type of compressor? We need to know. I suspect you are dealing with a centrifugal machine, but I am not going to guess when all you have to do is tell us.

The best manner to express your problem is to draw an accurate sketch of the 3-stage machine as you have described it - with the blow down valves, shut down valves, traps, and interconnecting piping. This way the problem can be discussed in detail and with accuracy.

How do you intend to calculate the time required for the blow down?

[shan]

Hi Art,

Thank you for your response. As indicated on the attached sketch, it is a reciprocating compressor with suction bottles and discharge bottles. I will use Hysys depressuring utility to calculate the time required for the blowdonw.

Regards

Shan

Attached Files

•  Compressor BD.xls   63KB   481 downloads

[Art Montemayor]

Shan:

Thank you for replying to my request. By doing so, in my opinion, you have helped yourself moreso in helping to resolve your query. By going back to the basics and identifying the type of compressor, you have revealed the set of conditions and factors that allow for an accurate and detailed response to your query. That is why I am so insistent on being specific and furnishing the basic details that form the background of your query. The answer to your query is as follows:

• Study the revised workbook that I submit for your review. Note the mechanical configuration of a typical reciprocating compressor. You have not stated if your cylinders are single- or double-acting, but I suspect they are the latter.
• The main difference between a centrifugal and a reciprocating compressor is that the latter has valves. These valves are an important factor to take into consideration when contemplating a blowdown of the machine. I always recommend that the manufacturer’s recommendations be strictly followed when doing a blowdown. You should follow those recommendations.
• Another factor you have failed to specify is WHAT type of blowdown. Is it an emergency blowdown? A routine shutdown blowdown? A startup blowdown? Is the machine running during the blowdown? All of these points have to be taken into consideration before recommending a “blowdown” procedure.
• Basically, if you are doing a shutdown blowdown on a reciprocating compressor, you will open the discharge blowdown valve first and immediately close the shutdown valve, downstream of the discharge checkvalve, next. You will also activate the suction shutdown valve – but not until after the machine has come to a dead stop. A multi-stage machine will normally “push” the process gas through the various stages in a sequential manner and the result will be that the machine is stopped without load – which is what you basically want to do.
• Note that a reciprocating machine depends entirely on the functioning of its suction and discharge valves. These are nothing more than sophisticated check valves that only permit flow IN ONE DIRECTION. Therefore, flow is allowed in the direction of higher pressure (towards the discharge of the machine) – but never “backwards” (towards the suction of the machine).
• You can “blowdown” a reciprocating machine while it is at a standstill – but the pistons should not be a major obstacle for the gas flow. This is done by allowing gas to freely flow from the suction port of the first stage to the final discharge valve of the final stage and on out through the discharge blowdown valve.

Based on the information and data you have supplied I believe you can depend on doing only one “blowdown” (what type I don’t know) through your final discharge blowdown valve. Again, to obtain a specific answer, you must supply specific details. Make sure that you allow for taking sonic flow into consideration and the potential this may hold for valve damage to your compressor. As I stated previously, you should be doing your blowdowns in conjunction with your manufacturer’s recommendations and instructions.

I hope this helps.

Attached Files

•  Compressor BD Rev1.xlsx   31.8KB   549 downloads

[shan]

Thank you Art for your time and efforts. Your advice is really helpful for me to better understand the compressor process.

Shan

[chemsac2]

Shan,

Art has essentially summed up everything. This is what I think of the system.
- Reciprocating compressors are not designed for settling out pressure as gas can not flow back through compressor suction and discharge valves when compressor trips.
- On compressor trip, shutdown logic closes both suction and discharge valves. Also, fail safe position for these is fail close and hence pressure equalisation would not be effected even on instrument air failure. I can not think of any other failure that would keep all suction and discharge valves open on compressor trip.
- Thus, I would design blowdown system for initial system pressure as last stage discharge i.e. 1000 psig. Many a times valves leak or get damaged. In such a case, pressure equalisation would be possible, but rate of gas flow through leaking valve would be much less than depressurization flow.
- Reciprocating compressor system needs to be designed for settle out pressure if capacity control in the form of spillback control valves is present.

Compressor valves as check valves:

I am not so sure about compressor suction and discharge valves being strictly check valves as stated by Art. For instance consider Chapter 1 of "Reciprocating compressors: operation and maintenance" by Bloch and Hoefner which has following description of valve slip:

"Valve slip means reversed gas flow through the valves before they have had time to seat at the end of the piston stroke. Obviously, this volume loss can occur through both intake and discharge valves. Minimum slippage occurs in a responsive valve; one that has minimum inertia so that the moving element can easily be controlled by air flow.

Slippage is usually much less through intake valves than through discharge valves. In the latter, differential pressure across the valve increases rapidly as the piston reaches dead center, so that if the valve does not respond instantaneously, high pressure gas naturally returns through the valve before it seats"

Regards,

Sachin

[Art Montemayor]

Sachin:

I wish I had the opportunity to discuss the workings and operation of a reciprocating machine together with all its components – mechanical and process related. However, the refusal to be specific on the part of original posters (OPs) does not allow me to comment on the important details. The OP was asked, and has not responded to my direct questions – as is often the case in our forums. Therefore, we don’t know WHAT type of blowdown is being discussed. It could be an emergency blowdown, a routine shutdown blowdown, a startup blowdown, or simply a venting. What is lacking also is the knowledge of the machine running during the blowdown.

I am afraid that without this knowledge your comments have no relevancy. Besides, some of your comments are not correct. For example,

• A reciprocating compressor system does NOT need to be designed for settle out pressure if capacity control in the form of spillback control valves is present. What you call a “spillback control valve” is, in reality, a recycle control valve located between the suction of a machine and its last discharge and is a means of controlling the capacity of the machine. This is the quickest and cheapest way to incorporate a capacity control capability in a reciprocating compressor. It is not related in any way to a need for settle-out pressure control. Settle-out pressure – as it applies and is understood – with respect to centrifugal compressors simply has no place or figures in no way to reciprocating process compressors. Even closed-loop, refrigerating reciprocating compressors and their controls take that into consideration. There IS a settle-out pressure in closed-loop, refrigerating processes, but this is of little or no concern in their design. The suction side of the machine is simply designed mechanically to withstand the maximum suction pressure(s).
• You are borrowing the term “valve slippage” without defining what exactly it means. It does not mean that reciprocating compressor valves are designed to leak backwards. In fact, I have used spare compressor valves as in-line check valves in gas plants that I have operated and supervised in the past. I learned this idea from an old gas plant superintendent in 1963. Some years later, a famous compressor valve manufacturer in Austria (Hoerbiger Valves) started to market their standard compressor plate valves as process gas check valves. This goes to show you the effectiveness of non-return characteristic features these valves have. So I see no relevence in your stating that some valves leak. They may leak – as check valves often do – but you can’t rely on when, how much, and under what circumstances they will. They certainly are not designed to leak. You state that you not so sure about compressor suction and discharge valves being strictly check valves. Well I can attest to field-proven tests and practice for many years that they are, indeed, check valves – and very efficient one at that.
• When you design a shutdown system for a reciprocating compressor it is VERY different from that of a centrifugal compressor. You may not want to have a suction shutdown valve to fail closed. You should always keep in mind the POSITIVE DISPLACEMENT characteristics of such a machine. These machines are perfectly capable of pulling a partial vacuum – in fact many were used to do just that some years ago. When you are handling low pressure hydrocarbon gases you certainly do not want to suck in atmospheric air through glands, seals, packings, etc.
• When dealing with reciprocating compressors, it is not wise to apply centrifugal compressor experience to what superficially “appears” to be the same application. One should have a complete and experienced knowldege of all the mechanical components and characteristics of reciprocating compressors before attempting to control or adjust them to process conditions.

I hope this helps to explain what I meant to include in my original post.

[chemsac2]

Art,

Thanks for the reply. I appreciate your insistence on not deviating from original post. In my previous post, I wanted to dwell on importance of construction of reciprocating compressors while designing blowdown system.

- Even with spillback control valve, system can see equalisation pressure (settle out pressure). In a similar 3-stage reciprocating compressor with spillback control valve for each stage (spillback from discharge to suction of same stage), I have seen suction PSV popping everytime compressor tripped. The moment compressor tripped, compressor logic closed both suction and discharge shutdown valves. This led to spillback from last stage discharge to first stage suction leading to some intermediate pressure. We may not need to size compressor suction for settle out pressure, but then suction KOD PSV has to be sized for settle-out flow. I was involved in commissioning of this three stage compressor and we had ascertained pressure equalisation as the cause of PSV popping.

- Agreed on the valve slippage. My concern was leaking valves leading to pressure equalisation.

- Yes that is very important consideration as well. Partial vacuum possibility had caused butterflies in my stomach when we were commissioning the system. We had a compressor trip by loss of open position (ZSH) of suction shutdown valve to partially counter it.

Reciprocating compressor is very complex and unforgiving equipment different from centrifugal compressor and understanding of mechanical construction is a must.

Regards,

Sachin