Page 166 - Reciprocating Compressors Operation Maintenance

1 52 Reciprocating Compressors: Operation and Maintenance

pressor. Low velocity of the gas is mandatory for effective liquid sep-
aration in suction vessels. In a few cases simply increasing jacket
water temperature or preheating the gas has eliminated condensing
out of corrosive elements.
Corrosion may not cause valve failure for several months. When it
does, it is best to change out and repair all the remaining valves,
because they are probably close to failure. When there is a corrosive
condition that cannot be eliminated, it is best to have a complete set
of spare valves. This will result in less downtime and less expense.

Abnormal Mechanical Action

There are four causes of abnormal operation, although there is some
interrelation between them:

L Slamming
2. Fluttering
3. Resonance or pulsation
4. Flow pattern

* Slamming can occur when a valve opens or when it closes. Valves
normally have little tendency to slam except possibly when the dis-
charge valve is opening.
Valve opening can be cushioned either mechanically, by springs or
by a gas cushion, or by both.
If a valve closes late, whatever the reason, backwash gas flow
rather than the springs will close the valve and slamming is certain
to occur. Slamming is suspected if the sealing element has a ham-
mered or mottled appearance where it contacts the seat. Listening
will often confirm slamming.
* Fluttering is a result of insufficient pressure drop through the valve. The
pressure drop is a function of velocity, density, and flow coefficient. If
springs are too stiff, a valve may be unable to open fully and, instead of
being held securely against its stop, it remains somewhere between zero
and full lift and oscillates. Springs usually suffer under these conditions.
In severe cases, a valve may strike the seat or guard several times
during one stroke of the piston. The inertia of the valve may cause it
to fully open against its stop, but pressure drop is insufficient to hold
it there. Consequently, it starts to close again.