Page 221 - Compression Machinery for Oil and Gas

210 SECTION II Types of Equipment

tube restriction between the two volumes in a volume-choke-volume acoustic
filter. As a result, the pressure at the volume side of the cylinder nozzle is near
constant pressure. The other part is a dynamic loss which occurs due to the ori-
fice plate on the cylinder flange plus the acceleration and deceleration losses
and pulsation of the gas in the bottle nozzle. The steady-state loss has the effect
of moving the inlet and discharge line on the PV card P s and P d by the amount of
the loss (0.5%–1.0% pressure drop is typical). The dynamic nozzle and orifice
loss appears on the PV card as a part of the valve loss. Taken together these
losses will typically add between 2% and 5% to the area of the PV card (power).

Cylinder Cooling Effects
Cylinder cooling continues to be embraced by the downstream (refining) indus-
try, where the upstream and midstream segments have fully embraced noncooled
cylinder designs. The origins of using of water jackets in reciprocating compres-
sors are somewhat uncertain. An article published in an 1891 edition of The Sci-
entific American states that water jackets were used in an effort to achieve near
isothermal compression in early air compressor designs. According to the article,
the subject very long stroke, very slow-speed compressors showed increased effi-
ciencies when cooling water jackets were utilized.
Cylinder cooling is required by API-618 such that the coolant temperature is
6°C above the inlet gas temperature to avoid the possibility of condensation.
This helps to minimize potential issues caused by corrosives in the process
gas and of possibly washing the lubricant away. This function is most useful
during compressor start-up, as the heat of compression will usually provide pro-
tection in operation.
Cylinder cooling is most effective for partially and fully unloaded cylinders.
Consider a cylinder partially unloaded by delayed suction valve unloaders. As
the volume efficiency is reduced the discharge temperature will rise due to the
throttling losses through the unloaded inlet valve and the reduced gas flow
available to remove the heat. In fact such an unloader system will normally have
a minimum flow below which the temperature in the cylinder becomes exces-
sive. However if water cooling is used the excess heat can be rejected to the
water jacket allowing lower permissible capacity before overheating occurs.
In a fully unloaded cylinder the throttling losses may cause the cylinder to
overheat unless the heat can be removed. Careful consideration of the unloaded
cylinder power consumption vs. the heat rejection capability is required to
determine if the cylinder will overheat. Cylinders unloaded by suction valve
unloaders typically will have an unloaded kilowatt of 5%–10% of the full-load
power. So consider a 750kW cylinder, if the unloaders result in 75 unloaded kW
then this is probably too much for a water jacket and the unloaded time must be
limited. However, if efficient unloaders are used and there is only 37 unloaded
kW then the water jacket may well be adequate to remove the heat within the
temperature limitations and long-term unloaded operating is acceptable.