Page 406 - Reciprocating Compressors Operation Maintenance

Reciprocating Compressor Calculations 391

The limiting rod loads are set by the manufacturer. Some manufactur-
ers require lower values on the rod in tension than in compression. For
this, the above limits are checked by reversing A HE and A CE. However,
for most applications, the compression check will govern.
Compressors below 500 psig, such as air compressors, are usually
sized by temperature. Rod load usually becomes the limiting factor on
applications above these pressures.

VALVES

Compressor valves are the most critical part of a compressor. General-
ly, they require the most maintenance of any part. They are sensitive both
to liquids and solids in the gas stream, causing plate and spring breakage.
When the valve lifts, it can strike the guard and rebound to the seat sev-
eral times in one stroke. This is called valve flutter, and leads to breakage
of valve plates. Light molecular weight gases, such as hydrogen, usually
cause this problem. It is controlled in part by restricting the lift of the
valve plate, thus controlling valve velocity. An earlier edition of API 618
specified valve velocity as:

Dxl44
(A-4a)

where V = average velocity in feet/minute.
D = cylinder displacement in cubic feet/minute.
A = total inlet valve area per cylinder, calculated by valve lift
times valve opening periphery, times the number of suction
valves per cylinder, in square inches.

At one time, compressor manufacturers objected to the above, because
it gave a valve velocity for double-acting cylinders one half the value
compared to equivalent single-acting cylinders. Therefore, manufactur-
ers' data on double-acting cylinders often indicated a valve velocity dou-
ble the API valve velocity. Care must be taken to find out the basis upon
which valve velocity is given.
The fourth edition of API 618 (June 1995) defines valve velocity as: