4 replies to this topic

[Giovanni I]

Good morning to everybody.
This is for me the first time i write in this forum.
I have a question regarding the lubrication of the reciprocating compressors. I have found a diagram where the parameters used to decide if lubricate or not are the flow rate and the discharge pressure (see attached file).
Do you know if there is any relation with the speed of the crankshaft or pistons and if yes, could you provide me any diagram or rule that can clarify to me this aspect?
Many thanks in anticipation.
Best regards.
Giovanni

Attached Files

•  Pages_from_RC__s.pdf   269.12KB   195 downloads

[Art Montemayor]

Giovanni:

Benvenuto to our Forum. Your query and request are practical ones that arise in the application of compressed air and industrial gases all the time. I just finished a project for two, 3-stage, reciprocating natural gas compressors. Each is driven by a 950 hp electric motor and both have oil lubrication in their cylinders. At this point, I want to make a point clear here: when we define a reciprocating compressor as “lubricated”, we mean that it has oil lubrication in its cylinders. All reciprocating compressors – by inherent necessity – have to be oil lubricated in their “power train”. Here, the power train is defined as all the running gear inside the crankcase (crankshaft, main bearings, crosshead, connecting rod, etc.). Normally the power train is lubricated by a lubrication pump that recirculates the required lubrication oil to the lubricated points, and to a cooler while pressurizing it.

That leaves three other critical sites that, in the past, always required oil lubrication of some sort:

1. The piston rod packing;
2. The cylinder valves; and,
3. The piston rings within the cylinder.

.All three sites pose potential oil contamination of the compressed gas and, therefore, have been the subject of scrutiny and safety engineers (due to the contamination and potential for explosion in the case of oxidizing gases). The application of special materials to the piston rod packing and the cylinder valves has virtually eliminated the need for lubrication at those sites – if one so desires it. However, the need for furnishing a non-lubricated piston has not been resolved 100% in some cases. Two types of resolution are presently available:

1. Use of a “labyrinth seal” design on the piston; and
2. Use of a “soft”, self-lubricated material instead of the customary cast iron piston ring.

Sulzer Brothers in Switzerland was the innovator of the labyrinth seal piston in their vertical, reciprocating compressors and this has met mixed reviews. As you can imagine, the machining of the piston in conjunction with the cylinder has to be super-accurate and precise. The clearances between the piston and the cylinder are microscopic and the piston virtually rides on a “gas” bearing. Any lateral movement or mis-alignment in a vertical machine causes a potential meeting of both piston metal and cylinder metal – with bad results. Additionally, as can be expected, there is always some “blow-by” of gas in the piston. In a double-acting cylinder, this means an inherent loss of capacity or efficiency.\

The use of re-inforced Teflon piston rings (and similar materials) has been more popular in non-lubricated compressor cylinders. The pistons actually have to be re-designed by using lighter metal (such as aluminum) for the piston and incorporating “rider bands” on the piston edges. The lighter piston makes it easer to support it in a true alignment and the rider bands serve as bearings to take some of the lateral forces away from the Teflon piston rings and allows these to act just as sealers and not as supports for the piston. This type of design has been successful but is subject to a range of compression ratios and discharge temperatures. Additionally, there is also more inherent piston ring leakage than in lubricated rings. Some applications have resorted to using a mixture of non-lubricated design together with some oil lubrication in the cylinder. This is called “mini-lube” and is what I furnished in the natural gas compressors mention above.

One elastomer material that has been introduced into reciprocating compressor design of late is the material Polyetheretherketone (“Peek”), also referred to as a polyketone. This material is now being used in compressor valves and has given excellent service in giving long working life and no need for lubrication at that point. PEEK is very expensive.

Piston speed in a reciprocating compressor is a subject that evokes a lot of emotion and opinions. This is so because manufacturers are always trying to lower the selling price of their compressors in order to remain competitive in the marketplace. To do this, they can increase the speed of a machine and obtain more capacity for less space occupied. However, the wear and tear of a reciprocating machine (in my opinion and also that of other experienced engineers) is directly related to the speed of the pistons. Therefore, an experienced engineer uses historical, empirical results to select an appropriate piston speed for a given application. In the industry, reciprocating compressors are rated as “slow speed”, “medium speed”, and “fast speed” machines. The actual, real values (in ft/min or meter/min) given to each of these classifications is rather clouded and not defined by anyone. It is a matter of subjective opinion on anyone’s part. I personally have always tried to limit my reciprocating compressor speeds to a maximum of 350 – 700 ft/min. I have found the machines that I have operated work better, longer, and more economically at that speed than others. That is my experience. Even the API 618 Standard, “Reciprocating Compressors for Petroleum, Chemical, and Gas Industry Services” fails to pin point a definite piston speed range limit for dependable service and wear. They tacitly admit to there being a practical limit to the maximum allowable piston speed – but don’t specifically identify it. By the fact that all in the industry know that API 11P standard is for “fast” (and quickly consumable) machines and API 618 is for more durable and reliable machines, we know that there is a difference in price and in service performance between the fast and the slower machines. Therefore, you must allow for this effect as an engineer.

Lubrication or non-lubrication depends on a variety of things:

• Lubricated cylinders tend to run cooler in the actual compression chambers. The oil not only “lubricates”, it also seals and it provides cooling for the affected surfaces.
• Sometimes you simply can’t inject oil into a cylinder. The case of Oxygen compression is an obvious example of this. Here, you are forced to use something like a soap solution instead. Breathing air is another example where you should use a non-lube cylinder – as well as instrument air.
• Non-lubricated cylinders represent less compressor effective life. They require more maintenance and periodic total replacement of rings, rider bands and sometimes rectification of pistons. They also are less efficient – capacity and power-wise.
• Compresion ratios are much more critical than the absolute discharge pressure as to whether you can use non-lubricated cylinders or not. The discharge temperature must be held to a relative lower value in order to protect the elastomer materials inside the cylinder. PEEK is an exception here, since it is more temperature resistant than Teflon. However, PEEK costs a relative fortune as compared to Teflon.
• Control of discharge temperatures is easily done by multiple staging on a machine. However, this means more capital costs – which is to be expected with non-lubricated features.

There are many other aspects to a non-lubricated reciprocating compressor, but I’ll save those for specific questions on the subject.

I hope this helps you out.

[Giovanni I]

Many thanks for the time you spent answering to my question.
I apologize because, as you noted, my question was not so clear, but you understood well that I was asking about the pistons lubrication.
You gave me more details I could expect so thanks again.
Today, I had the opportunity to discuss with a colleague of mine who as 20 years of experience about this issue and he showed me a diagram where are shown limits for non lubricated machines in terms of pistons speed (> 4.5 m/sec) and discharge pressure (>100 bar a). From your experience, is it ok? If yes, is it valid also for air compressors (discharge pressure 80 bar a)?
Best regards
Giovanni

[Art Montemayor]

Giovanni:

No apology is needed. I went into the explanation of the various lubrication systems not for your sake so much as for the sake of the other Forum readers who are unfamiliar with reciprocating compressors.

As I said, the subject of when and where you apply non-lubricated cylinders is varied and complex. It mostly depends on the end-user (or operator). You, as the user, know more about the application and the necessity or importance of applying a non-lubricated cylinder to your process and you should make that decision. Do not depend on anyone else – especially a diagram drawn by someone we don’t know or for a reason we know little about – to do that for you. For example, you mention an air compressor discharging at 80 bara, but you don’t state the application. If the air is for breathing purposes or for critical instruments, you could be forced into a non-lubricated cylinder use. For 80 bara you would probably require a 3-stage machine – maybe even a 4-stage model if the discharge temperatures have to be maintained low.

The smartest engineering decision is to always employ oil lubrication in your cylinders if you can. The reason for employing non-lubricated design should be because the oil simply can’t be tolerated – be it for process reasons, safety reasons, or because of contamination. Don’t forget that you can always apply a solid bed adsorbent on the discharge of any compressor if you need very dry gas. And when you apply the adsorber, you will naturally also remove essentially all of the oil in the gas stream as well. So, even if you use oil lubrication, you can subsequently remedy the contamination issue in most cases.

My idea of a good design piston speed is around 2.5 mt/sec. I regard a piston speed of 4.5 mt/sec as “high speed”. But as I said before, this is being subjective and depends on one’s experience.

I hope this helps you.

[Art Montemayor]

Giovanni:

Further on our subject of compressor speeds, a colleague stepped into my office after I wrote my last comments to you and started in on the same subject. He brought in a copy of a book: "Compressors - Selection and Sizing", 2nd Edition, by Royce N. Brown and published by Gulf Publishing Co. I thought you might be interested in what Mr. Brown says about the subject and I quote him directly:

"For a single-acting cylinder compressing at the outer end of the cylinder,

Pd = (St) (N) ( Π D²/4)....................3.1

Pd = (St) (N) [ Π (D² –d²)/4)............................3.2

Where,

Pd = Piston displacement, cubic inches
St = Piston stroke, inches
N = Compressor rotative speed, rpm
D = Cylinder internal diameter, inches
d = Piston rod diameter, inches


Piston Speed

Another value to be determined is piston speed, PS. The average piston speed may be calculated by:

PS = 2 x St x N .....................................3.8

The basis for evaluation of piston speed varies throughout industry. This indicates that the subject is spiced with as much emotion as technical basics. An attempt to sort out the fundamentals will be made. First, because there are so many configurations and forms of the reciprocating compressor, it would appear logical that there is no one piston speed limit that will apply across the board to all machines. The manufacturer is at odds with the user because he would like to keep the speed up to keep the size of the compressor down, while the user would like to keep the speed down for reliability purposes. As is true for so many other cases, the referee is the economics. An obvious reason to limit the speed is maintenance expense. The lower the piston speed, the lower the maintenance and the higher the reliability. The relationship given by Equation 3.1 defines the size of the cylinder. Therefore, if the speed is reduced to lower the piston speed, then the diameter of the cylinder must increase to compensate for the lost displacement if one is to maintain the desired capacity. As cylinder size goes up, so does the cost of the cylinder. It is not difficult to see why the user and manufacturer are at somewhat of a cross purpose. If the user's service requires a high degree of reliability and he wants to keep cylinder and ring wear down, he must be aware of the increase in cost.

To complicate the subject of piston speed, look at Equations 3.1 and 3.8. Note the term St (stroke). The piston speed can be controlled by a shorter stroke, but because of loss of displacement, the diameter and/or the speed must be increased, If only speed is increased, the whole exercise is academic as the piston speed will be back up to the original value.

If, however, diameter alone or both diameter and speed are increased, the net result can be a lower piston speed. This is another factor that comes to bear at this point because valve life decreases with the increase in the number of strokes and this can negate the apparent gain in maintenance cost by shortening the stroke (but increasing the RPM). It would appear that the engineer trying to evaluate a compressor bid just can't win. The various points are not tendered just to frustrate the user but rather are given to help show that this is another area that must have a complete evaluation. All facets of a problem must be considered before an intelligent evaluation can be made.

After all the previous statements, it would seem that it is very difficult to select piston speed. For someone without direct experience, the following guidelines can be used as a starting point. Actual gas compressing experience should be solicited when a new compressor for the same gas is being considered. These values will apply to the industrial process type of compressor with a double-acting cylinder construction. For horizontal compressors with lubricated cylinders, use 700 feet per minute (fpm) and for non-lubricated cylinders use 600 fpm. For vertical compressors with lubricated cylinders, use 800 fpm and for non-lubricated cylinders use 700 fpm. Another factor to consider is the compressor rotative speed relative to valve wear. The lower the speed, the fewer the valve cycles, which contribute to longer valve life. A desirable speed range is 300 to 600 rpm."

I think Mr. Brown has been reading some of my threads in the past. His comments are almost identical to mine - as well as his experience. This does not surprise me.