5 replies to this topic

[Novice Instrum]

Good day to all,
I am doing a basic engineering for C2 compression facility whose design capacity is 20000 Kg/hr. This is my first project involving with reciprocating compressor.
I have attached basic scheme herewith. The abbreviation used are: SDV: Shutdown valve, PC: Pressure Controller, D-001: Suction Drum, K-001: Reciprocating Compressor, E-001: Air Cooler, PCV-001: Spill back valve.
The feed is received from other party, and at a pressure 34 barg @ 50 degC at our battery limit. The compressor is single stage compressor to boost up the pressure to 67 barg.
My process engineer given me some condition for the compressor spill back valve based on pressure.
The spill back valve should open when the inlet pressure is less than certain value to avoid pressure low-low trip of compressor.
Similarly the if the discharge pressure increases above certain value due to downstream facility shutdown, the spill back valve should open to recycle.
Flaring should be avoided, so it is not recommended to flare if discharge pressure goes high.

My query is, Is it possible to have such pressure control as per the scheme attached and description above. Which selector function to use to select between PC-1 (Suction) & PC-2 (Discharge)?
For me other simple solution is to control spill back valve based on the suction pressure and provide new control valve to relieve the excess flow based on the discharge pressure.

Please advise. If some data are missing, please let me know.

Attached Files

•  C2 Compression Scheme.pdf   20.63KB   576 downloads

[Art Montemayor]

Novice:

You are a very lucky rookie engineer since you have been given a reciprocating compressor to control. It could have been a centrifugal – which is difficult and worse to control. A reciprocating compressor is easy to control with respect to capacity. First of all, it is an inherent metering tool since it is a positive displacement device – which means that its capacity at a fixed rotational speed is fixed and shouldn’t vary (theoretically). But it will vary – although very slightly. You should first familiarize your self with what is in front of you: a positive displacement device whose capacity can be varied in a number of ways:

• With Clearance Pockets;
• With suction valve unloaders;
• With plug suction valves;
• With the speed of the driver;
• Varying the suction pressure source;
• With a recycle capacity control valve (what you call a spill back valve”).

What capacity control method you apply depends on a variety of basic scope reasons and what size and type of compressor you have:

• Does the compressor have built-in clearance pockets? This is the best and most desirable method.
• Do you want to save work energy?
• Do you just want a down-and-dirty, cheap method? This is a recycle capacity control valve.
• What kind of driver do you have? Can you vary its speed?
• Can you change your suction valves to plug type?
• How is your ethylene source controling the fed suction gas capacity? Can you suddenly apply your shutdown without advising your source?
• Will you be varying the ethylene capacity downstream by using the compressor to accomplish this?

I hope you know enough about reciprocating compressors to understand what I am referring to. If you don’t, then I strongly recommend you set yourself to learn as much as you can about what you are attempting to control. This is a basic and required step before you go any further. In order to apply instrumentation and controls to a positive displacement device with success and safety, you must fully understand what you can – and you can’t – do.

Your sketch is difficult to interpret. I will create one for you on Excel so that comments and “call outs can be attached to it and it can serve as a communications tool. I will attach it to this post as soon as I finish it.

Attached Files

•  Ethylene Compressor Capacity Control.xls   1016KB   542 downloads

[Novice Instrum]

Thanks you very much for quick response and efforts. I am clearly stating some of the project objective below.

1. Reduce (Eliminate) the flaring, now everything is flared.
2. The gas 20000 Kh/hr is less than 5% of the end user capacity.
3. Client want the cheapest solution since for him this gas recovery doesn't add any value addition, but only to comply new local rule and regulation for flaring.

Now following are the answers to your question
[*]Does the compressor have built-in clearance pockets? This is the best and most desirable method.
Ans: Not sure, We are consulting with the vendors and we can ask for it.
[*]Do you want to save work energy?
Ans: Not a priority.
[*]Do you just want a down-and-dirty, cheap method? This is a recycle capacity control valve.
Ans: Yes, definitely a cheap method is preferred.
[*]What kind of driver do you have? Can you vary its speed?
Ans: Electrical. Variable speed drives can have, but again will prefer to avoid to have cheap solution.
[*]Can you change your suction valves to plug type?
Ans: May be possible (Basic Engineering stage), to discuss with the vendor.
[*]How is your ethylene source controling the fed suction gas capacity? Can you suddenly apply your shutdown without advising your source?
Ans: Upstream is properly controlled, they have flow control and pressure control systems. In any event we shutdown our unit or reduce consumption, still they can divert the gas to fuel gas network, they can reduce the flow, or even they can divert to other users.
[*]Will you be varying the ethylene capacity downstream by using the compressor to accomplish this?
Ans: No

I dont want to shut the unit, when any upset in upstream unit or downstream unit. In both cases I just want to recycle.

As per the scheme, I can control the effects by upstream upsets.
No I would add one more pressure controller at compressor discharge side after the cooler, who will act on the recycle valve. I will put a selector function to select between suction & discharge pressure controller to act on the recycle valve. I am not sure to give priority to which controller,will it work, what can go worng, is it a stupid idea at all etc.
Otherwise I will add one more control valve at compressor discharge side and the new pressure controller at discharge side will act on it to send the excess gas to fuel gas network.

I hope I am clear now, and attached revised (Rev 1) scheme.

Attached Files

•  Ethylene Compressor Capacity Control_Rev 1.xls   322KB   268 downloads

[Art Montemayor]

Novice:

And thank you for the prompt and detailed reply. With this additional information I believe we can go directly to the grain of the application and not waste any time and effort, risking mistakes and/or misconceptions.

Please refer to the attached workbook. I am having trouble downloading your Rev1, so please use this information and move it into your Rev 1 copy, making it Rev 2. I will do the same as soon as I download your Rev 1. You will note that the listing of the thermophysical properties of Ethylene starts to reveal several important things about your system. I suspect your gas stream is really an impure ethylene stock, but since we don’t have the composition or the related thermophysical data, then I have to use pure ethene properties.

Note that at the discharge conditions of 68 bara and 60 ^oC, you have a supercritical fluid (SCF) – a fluid that behaves like a liquid AND a gas; it is neither. Now, both you and I can appreciate that you don’t want to cool this discharge SCF too much because you run the danger of expanding through the recycle control valve and creating a mixed liquid and vapor product and you don’t want any liquid possibly going into the suction of a reciprocating compressor. You can’t do that without running the risk of causing a catastrophic break-up of the cylinder. Therefore, the application isn’t all that direct and simple. You must take into consideration the thermophysical properties of the fluid pre- and post-expansion through the recycle control valve. What you want is a recycle control valve to function as the basic capacity control for the compressor – and this can be done in a simple manner, just as I have drawn it out in my PFD in the workbook. You do not need (nor want, in my opinion) a selector switch to change signals and try to “control” the discharge pressure. You should control the discharge pressure – if you want or require a constant discharge pressure – by using a simple back-pressure control valve on the compressor (if your process downstream isn’t or can’t do this).

However, to have the recycle control valve control the capacity of the compressor, you must control (or hold steady) the suction conditions into the compressor cylinder – and this will “fix” the discharge temperature (since you are holding the discharge constant as I recommend above). This is a cumbersome and simple problem that is a trade-off for using a simple and cheap capacity control method. You want the recycle valve to handle SCF, but produce a gas at 35 bara that is compatible with the mechanical ratings of the compressor cylinder – and yields a discharge temperature that is suited for recycling back to the suction. That is what you imply in your asserted scope of work: you want the ability to recycle 100% of the compressor’s capacity in order to avoid shutting it down during periods when the gas supply is cut off or you suffer downstream shutdown(s). This is an important part of your scope of work and the recycle valve can accomplish it – but at a price (as it is expected). Whoever specified a 60 ^oC discharge temperature entering the recycle valve was probably doing the same kind of logic that we are here. However, as noted on the Thermo data, the valve expands the SCF in an isenthalpic manner and the outlet temperature out of the CV is approximately 30 ^oC – a temperature that is hotter than the required discharge of 60 ^oC to continue the recycle at 100% turn-down of the capacity. That is what I have initially suspected and why I show an after cooler on the compressor discharge. You can “trial-and-error” the discharge temperature to find the one that satisfies the suction conditions, using the thermo data I supply. But the point here is that you should know the sensitivity of the system and what zones you are designing for. You may – or may not – require an aftercooler.

Since we don’t know the composition, note that I show inlet and outlet separators. These are important because I don’t know if there are condensables included in the ethene gas. As I have stated, it is of grave importance to protect a reciprocating compressor from any included liquids in the gas and, for that reason, separators are always part of the total design. Note that I use a separator on the discharge as well. This is to protect the CV from receiving any liquid – and may also be required to protect the downstream facilities (whatever they are). Note specifically where I take the recycle gas and where I inject it into the suction part of the compressor. This is done this way for a variety of obvious reasons.

I have not shown any required safety relief valves, and I leave that up to you as well as the recognition that THERE SHOULD OCCUR A HAZOP immediately after you have concluded the preliminary design and produced a preliminary P&ID. I believe you recognize the importance of this critical step.

Recapping all that I have stated:

• You should be able to control the compressor’s capacity 100%, using a recycle control valve designed for that purpose. However, you must design for the appropriate discharge conditions that allow for a constant, maximum suction temperature that will, in turn, allow for the desired discharge temperature.
• You should also capitalize on the above ability and design the startup and shutdown of the compressor under NO-LOAD conditions by using the unloading capability. This should ensure 100% ethene containment, without any need for flaring or venting when starting up and shutting down.
• You will be consuming 100% of the required electricity to compress 100% of the design gas capacity – 100% of the time, whether you are on-stream or just idling off-stream. That is one of the big trade-offs in applying a recycle capacity control valve to a compressor.

Attached Files

•  Ethylene NIST Thermophysical Data.xlsx   116.95KB   309 downloads

[Novice Instrum]

Art:

Thank you very much for the detailed explanation. The issue of SCF, was discussed with our process engineer, and he is working on it.
Yes we will doing HAZOP, in fact this question was raised during one of our internal pre-Hazop review.

I don't want to control discharge pressure, or to maintain constant discharge pressure by using back pressure control valve.

My requirement is simple (I believe so), Suppose I am running on any capacity (be it 100%, 75, 50) and If the downstream Shutdown Valve start closing for any reason, I could be able to recycle back the flow before activation of discharge PAHH trip, or before my PSV pop up.

How this can be achieved?

Regards

Novice

[Art Montemayor]

Novice:

The basis of the recycle capacity control valve, as I’ve drawn it on my PFD, is to maintain a positive, constant suction pressure to the 1st stage of the compressor. By doing this, what you essentially are doing is fixing the conditions (pressure & temperature) entering the 1st stage and this “fixes” the capacity of the 1st stage to a constant value and this maintains a constant flow rate through the compressor – but not necessarily to your discharge target. All the recycle valve is doing is making up for any deficiency of gas coming from your source. The NET compression gas product going to your target is equal to what your source is feeding your compressor. That is how this version of capacity control works. It compensates for the lack of sufficient source gas getting to the suction. It can't compensate for an excess of source gas.

If you set the recycle valve to function on excess compressor discharge pressure (due to a diminished need or consumption at your target – or a plug up or hazard shutdown downstream – the valve will open to maintain a set pressure (that you select). When this happens, the compressor will recycle the amount of gas that it requires to maintain the set pressure – which means that in the least case the amount of gas used from your source will equal the amount that is being consumed or used in your target. However, the worse case is a complete shutdown downstream, and this forces the compressor to reject any gas coming from your source because you now have a 100% recycle around the compressor. This is OK for your compressor (assuming you are cooling all recycle), but your source gas will either accumulate (and raise the suction pressure) or it will have to be automatically shutdown to avoid a high suction gas pressure condition at the compressor suction.

This control scheme works well with a reciprocating compressor; the problems that you face are not with the compressor, but with your gas source and your gas target. You should carefully study the potential hazards that can evolve or appear and instrument accordingly to mitigate a potential danger. A sudden blockage in the compressor discharge line will cause a very rapid and sudden pressure increase in the discharge system. You have pressure relief valves to protect you from this dangerous situation – but they are not meant to CONTROL the situation. In order to control such a situation you must have sufficient time to activate the recycle valve and this may require a surge drum in the discharge together with HIPPS type valves and a HIPPS control system. Don’t forget that you must block the source gas as well as the discharge target. The positive side to all this is that you are able to comply with your basic scope of work: avoid any flaring or venting to atmosphere and keep the compressor running safely loaded.

I hope I have responded to your specific question directly and you have a good grasp on what you must do to fulfill your scope.