Page 302 - Compression Machinery for Oil and Gas

Screw Compressors Chapter 6 287

check valve at the discharge of the compressor closes. The pressures at com-
pressor discharge and suction equalize after a few seconds at the SOP. The com-
pressor should come to standstill only after the pressures have equalized in order
to prevent reverse rotation. Typical run-down times may range from a few sec-
onds for small high-pressure compressors up to approximately 1min for large
compressors operating in vacuum service. The compressor seal system must be
able to operate at the SOP and must adjust to the SOP within seconds in order to
avoid malfunction of the suction end seals and possible process gas leakage. For
compressors with a built-in volume ratio>2.5 the torque increases with increas-
ing suction pressure. A massive increase of suction pressure may lead to over-
loading the compressor and coupling. Therefore it is important that the SOP is
as close to the suction pressure as practical. The SOP depends on suction pres-
sure, discharge pressure, and the gas volumes at suction side and discharge side
up to the check valve.
For start-up and shutdown the discharge side gas volume should be as small
as practical and the suction end volume should be as large as practical. Check
valves in the suction line must be avoided. Block valves in the suction line
should be closed only after the pressures have equalized at SOP. If all these
measures are not sufficient to reduce the SOP to an acceptable level a quick
opening and sufficiently sized blow down valve to a flare should be placed
in the discharge line.

Stopping of Flooded Screw Compressors and SOP

Oil-flooded screw compressors must also be unloaded during coast down to
stop, to protect the bearings. However, it is also important to avoid a rapid
depressurization of the discharge side. The oil in the bulk oil separator, any sec-
ondary separator vessels, and everywhere in the oil system is at discharge pres-
sure or higher, with some amount of process gas dissolved in the oil. Rapidly
reducing the pressure in the system will cause the gas to rapidly bubble out of
the oil, which can lead to foaming of the oil (champagne bottle effect). It can
take many hours for the foam to return back to liquid form. If undetected, foam-
ing in the oil can damage the compressor if the system is running. The rapid drop
in the pressure can also cause oil migration toward the lower pressure area,
resulting in a total loss of oil from the bulk oil separator vessel. It is important
to have a check valve on the suction side as well as the discharge side, and for
the suction check valve to be as close as possible to the compressor. This
ensures that the SOP is only slightly lower than the discharge pressure. During
coast down, the slide valve should be moving quickly to the unloaded position
to protect the bearings from high load at low speeds. If the recycle valve con-
nects downstream of the suction check valve, it can be opened during coast
down. Otherwise, it should remain closed. After the compressor has stopped,
if it is necessary to reduce the system pressure, it should be vented slowly to
prevent oil foaming.