2 replies to this topic

[Fr3dd]

Hello everbody,

I have a reciprocating compressor that handles nitrogen in the CCR regeneration section. EPC contractor states in the Material Requisition that API STD 680 shall be followed. I thought that API STD 680 was used only for plant and instrument air packaged systems. Could anybody please clarify that?
The main issue is regarding the maximum allowable discharge temperature. According to API STD 618 maximum discharge temperature is 300°F for any service, and for non-lubricated services that temperature should be lower. However, according to API STD 680, 360°F is the maximum temperature for non-lubricated cylinders. Which standard should i follow in this case?

Thanks in advance,

Regards,

[Art Montemayor]

Fr3dd:

In the past I have confronted this apparent dilemma and I have always come to the conclusion that the principal issue is the cost of the capital expenditure. This is an unfortunate and often fatal decision that a field engineer has to face.

The whole problem can be resolved by the simple question(s):

• How much money are you limited to spend?
• How critical is the service?
• Can you tolerate a higher than normal demand on maintenance and associated operating costs?

You failed to tell us all the necessary details about your nitrogen compressor – such as number of stages, type and efficiency of intercoolers, total horsepower, lube or non-lube service, type of unloading, etc, etc. Because you don’t give us all the details and basic data, I can’t give a specific answer. However, I can make a general comment or two on the unwise decision to rely on low-capital cost reciprocating compressors (usually working at high piston speeds, high compression ratios, high rod loadings, high discharge temperatures, and high valve velocities. My recommendation would be: DON’T.

You are perfectly correct in challenging the lack of importance given to the maximum allowable discharge temperature. This is directly related to the correct and recommended compression ratio. Usually API 680 compressors will be running at 1,000+ rpm speeds with piston speeds well above the norm. These type of “packaged” compressors are designed for a limited operating life. They literally will wear themselves down. They are cheap, consumable, and inefficient. Their application lies in the area of low first capital cost and high operating cost – sometimes linked to safety hazards.

If you have a serious, invested process that must be relied upon to perform daily and consistently, then I would apply only the API 618 Standard. You pay more in capital expense, but you have a rugged and reliable, safe machine.

[Fr3dd]

Glad to see you again, Mr. Montemayor.

I'll give you a more detailed description of this unit. As I told you, service is nitrogen; mass rate is 2476 lb/h; compression ratio is 2.9; there is only one stage; there is two (1 operating + 1 spare) reciprocating dry piston; the estimated power is 66 HP; the assumed adiabatic efficiency is 0.80; assumed adiabatic head is 39398 ft; and the discharge temperature shown on process data sheet is 316°F (Cp/Cv = 1.407).

Checking the values I figured out that for temperature calculation, the efficiency was not taken into account. They based on ideal behavior and used the equations shown on GPSA which are commonly used but not recommended. Using the efficiency and the equation (also shown on GPSA) the discharge temperature is around 370°F. How far am I from actual discharge temperature? Which is the best way to address this kind of services?

Also, the service is critical; this unit compresses nitrogen gas used for catalyst lift from regenerator to first reactor and from last reactor back to regenerator which is continuous and very important part of the process.

If there is any other information you need, please let me know. And many thanks for your help.

Regards,