ARNOLD, K. (1999). Design of Gas-Handling Systems and Facilities (2nd ed.) Episode 2 Part 4 pdf

Reciprocating Compressors 311

Single-acting cylinder, crank end

Double-acting cylinder

where RLC = rod load in compression, ib

RL, = rod load in tension, Ib

ap = cross-sectional area of piston, in.2

Pd = discharge pressure, psia

Ps = suction pressure, psia

Pu = pressure in unloaded area, psia

ar = cross-sectional area of rod, in,2

The calculations shown above provide the gas load imposed on the rod

(and crosshead bushing) by the compressor cylinder piston. To provide a

reasonable crosshead pin bushing life, the rod loading at the crosshead

bushing must change from compression to tension during each revolu￾tion. This is commonly referred to as "rod reversal" and allows oil to

lubricate and cool one side of the bushing while load is being applied to

the other side of the bushing.

A single-acting, head end cylinder will not have load reversal if suc￾tion pressure is applied to the crank end. Similarly, if discharge pressure

is applied to the head end of a single-acting, crank end cylinder, load

reversal will not occur.

In addition to the gas load, the rod and crosshead pin bushing is sub￾ject to the inertia forces created by the acceleration and deceleration of

the compressor reciprocating mass. The inertia load is a direct function

of crank radius, the reciprocating weight, and speed squared. The total

load imposed on the crosshead pin and bushing is the sum of the gas load

and the inertia load and is referred to as the "combined rod load."

The combined rod load should be checked anytime the gas loads are

approaching the maximum rating of the compressor frame or anytime rod

reversal is marginal or questionable.

312 Design of GAS-HANDLING Systems and Facilities

COOLING AND LUBRICATION SYSTEMS

Compressor Cylinder Cooling

Traditional compressor cylinder designs require cooling water jackets

to promote uniform distribution of heat created by gas compression and

friction. Some of the perceived advantages of water-cooled cylinders are

reduced suction gas preheat, better cylinder lubrication, prolonged parts

life, and reduced maintenance.

Operating experience during the last 30 years has proven that com￾pressor cylinders designed without cooling water jackts (non-cooled) can

successfully operate in most natural gas compession applications. Some

of the perceived advantgaes of non-cooled cylinders are simplified cylin￾der designs that reduce cost and improve efficiency, reduced initial sys￾tem costs due to reductions in the cooling water system, improved valve

accessibility, and reduced weight.

Many manufacturers, users, and compressor applications still require

that compressor cylinders be supplied with liquid-cooled cylinders. Fig￾ure 11-21 includes schematics of several types of liquid coolant systems.

In static systems, the cooling jackets are normally filled with a glycol

and water mixture to provide for uniform heat distribution within the

cylinder. This system may be used where the AT of the gas is less than

150°F and discharge gas temperature is less than 190°F.

Thermal siphons use the density differences between the hot and the

cold coolants to establish flow. This system may be used where the AT of

the gas is less than 150°F and discharge gas temperature is less than 210°F.

Forced coolant systems using a mixture of glycol and water are the

most common for natural gas compressors. Normally, the compressor

cylinder cooling system and compressor frame lube oil cooling system is

combined. A single pump is used to circulate the coolant through the

cylinders and the lube oil heat exchanger and then to an aerial cooler

where the heat is dissipated.

When forced coolant systems are used, care must be taken to provide

the coolant at the proper temperature. If the cylinder is too cool, liquids

could condense from the suction gas stream. Thus, it is desirable to keep

the coolant temperature 10°F higher than that of the suction gas. If the

cylinder is too hot, gas throughput capacity is lost due to the gas heating

and expanding. Therefore, it is desirable to limit the coolant temperature

to less than 30°F above that of the suction gas.