4 replies to this topic

[pvragesh]

Hi Everyone,

It was great to read all the posts so far.

We have an application for Non-Lube Instrument Air compressor to be supplied in accordance with API-618.

Solution is a two stage compressor with Adiabatic discharge temperatures of 159 Deg C at each stage.Piston speed restricted to 3.5 m/s. PPS/PTFE wear rings are supplied.

In case API-618 is to be complied with , we may have to go for a three-stager, which means additional cost (beyond our budget)

Could some one help me with arguments in favour of the two-stager(discharge temperature being the key issue) ???this would help lift the project out of the doldrums.

Thanks in anticipation

Ragesh

[Art Montemayor]

Ragesh:

This is a very important issue and I have discussed and explained a lot of things with regards to it in these and other Forums in the past. The basic data you’ve given is a start; however there is more information that needs to be given before a final decision is made on the correct compressor you need. But what you’ve given allows us to give you a “push start” on your project.

To show you how I reach an engineering decision on the characteristics of the air compressor you are describing, I would have to generate an Excel workbook with calculations and illustrations of how one goes about deciding and calculating the requirements for compressing instrument air efficiently and safely with a reciprocating compressor. But first, allow me to quickly on some important points you have mentioned and which have a big impact on your final decision:

• I strongly recommend that you NOT apply the API 618 Standard on an instrument air compressor. I realize that I am ignorant of the industry you are in and the capacity of the machine you are proposing, but the API Standard was meant for different applications. API 618 covers the minimum requirements for reciprocating compressors and their drivers used in petroleum, chemical, and gas industry services for handling process air or gas with either lubricated or non-lubricated cylinders. It is not intended for utility services, such as instrument air. I am not implying that it can’t be used for your application, but the amount of excess and extra expense that you will incur upon applying this Standard will increase the cost of your compressor astronomically – and that is assuming that you find a manufacturer that will build a utility compressor to those standards. Chances are that you probably won’t find a manufacturer who will comply with what you are asking. The reason(s) is that the market for air compressors is very competitive and manufacturers strive to build very “lean” machines – without any of the details that API 618 demands. If you insist on applying API 618 you will inherit a lot of expensive extra costs.
• If you are indeed specifying an instrument air compressor, you are probably compressing atmospheric air to a pressure of 150 psig – as a MAXIMUM. Am I correct? If so, then I commend you on selecting a 2-stage reciprocating model (which I assume you mean to state). For an over-all compression ratio of 11 (165/15) and employing non-lubricated cylinders with reinforced Teflon rings, a 2-stage machine is the MINIMUM indicated model to apply. A single stage machine will not last long and is definitely not suited to the specified work conditions.
• If you are compressing atmospheric air to 150 psig through a 2-stage machine, you will have approximately a compression ratio of 3.31 on each stage. The discharge temperature on each stage (assuming 90 ^oF as the suction temperature on each stage) will be approximately 315 ^oF. I consider this as an excessive, steady discharge temperature for a Teflon ring compressor. I normally design for a discharge temperature of 250 ^oF and will accept 300 ^oF as the actual, maximum discharge on oil-lubricated cylinders. For non-lubricated, I prefer to specify 250 ^oF and settle for a maximum of 275 ^oF if I have to. I do not accept an oil-lubricated air compressor with a discharge temperature of 350 ^oF or more. This, in my opinion is bordering on a very hazardous air explosion situation. A non-lubricated cylinder is a good, safe answer to the explosion hazard, but it applies an elastomer (Teflon) that is susceptible to heat degradation and deformation. That is why I limit the temperature on Teflon rings as I do.
• The piston speed that you mention (3.5 m/s) is a little fast for a non-lube cylinder, in my opinion. I normally apply a maximum of 500 ft/min (2.54 m/s) as the piston speed for a non-lubricated cylinder reciprocating machine.
• You do not mention your steady, maximum discharge pressure or your compression ratio, so that we don’t know the critical sizing data for determining the best number of cylinders. I have to presume you are going to use atmospheric air as the feed and possibly dry the discharge product either through a refrigeration unit or an adsorption dryer. If you are intending to compress to 150 psig, then I would apply a 3-stage, non-lubricated cylinder unit for steady, continuous duty.
• You also don’t give us the capacity of the machine and therefore, we don’t know whether it is a vertical or horizontal unit, a balanced-opposed, or an in-tandem unit.

I await your response and comments.

[pvragesh]

Thanks a ton for your pointers.

I totally agree with your views regarding the applicability of API-618 on instrument air compressors.

But our technical committee has already specd this in and we had a tough time trying to convince them against going for API-618 compliance

The machine is a balanced opposed configuration with 18 inch cylinder on the first stage and 10 inch cylinder on the second stage.

Inlet Pressure =1.03 Bar A
Discharge pressure at second stage =8 bar A
Inlet temperature to first stage=45 Deg C
Inlet temperature to second stage =52 Deg C
Flow =1100 Nm3/hr

The site being remote ,there is scarcity of constant water supply and we have to employ air coolers for interstage and cylinder jacket water cooling.

Kindly advise.

[Art Montemayor]

Ragesh:

If you are only compressing atmospheric air to 7 barG and the total capacity is only 1,100 Nm³/hr (approx. 1.0 MMScfd), and you want to keep your instrument air devoid of oil - something that I support - then, why don't you just purchase a small centrifugal air compressor? Of course, I'm assuming that you will have a steady production of air (making the centrifugal more practical and not likely to surge or suffer capacity turn-downs.

For the conditions you are now describing, I would seriously favor a centrifugal compressor - especially in light of the non-lube requirements and the scarcity of cooling water. But, the capacity load should be as steady and constant as possible.

If you are still determined to install a reciprocating machine for this application, then I would insist on a 3-stage machine - primarily due to the relative high suction temperatures specified. You have to protect the Teflon rings and rider bands from excessive discharge temperatures and since you don't have cooling water, then your only alternative to this constraint is to lower your stage compression ratios by adding another compression stage. This adds more to the capital cost and, I suspect, makes the centrifugal much more attractive.

[JoeWong]

Thanks a ton for your pointers.

I totally agree with your views regarding the applicability of API-618 on instrument air compressors.

But our technical committee has already specd this in and we had a tough time trying to convince them against going for API-618 compliance

The machine is a balanced opposed configuration with 18 inch cylinder on the first stage and 10 inch cylinder on the second stage.

Inlet Pressure =1.03 Bar A
Discharge pressure at second stage =8 bar A
Inlet temperature to first stage=45 Deg C
Inlet temperature to second stage =52 Deg C
Flow =1100 Nm3/hr

The site being remote ,there is scarcity of constant water supply and we have to employ air coolers for interstage and cylinder jacket water cooling.

Kindly advise.

What is the polytropic efficiency at operating point and it maximum reported efficiency in compression curve for both stages ?