The seal oil system of a centrifugal compressor shown can be divided into four major sub-system: The supply syste, the seal housing system, the atmospheric drain system, and the seal leakage system
The seal supply system consists of the reservoir, pumping units, exchangers, transfer valves, temperature control valves, and filters. The purpose of this sub-system is to continuously supply clean, cool sealing fluid to the seal interfaces at the correct differential pressure.
The seal housing system comprises two different seals; a gas side bushing, and an atmospheric bushing, and an atmospheric bushing. The purpose of the seal housing system is to positively contain the liquid in the compressor and not allow leakage to the atmosphere. The seal fluid is introduced between both seal interfaces thus constituting a double seal arrangement.
The purpose of the gas side bushing seal is to constantly contain the reference fluid and minimize sour oil leakage. This bushing can be conceived as an equivalent orifice. This concept is similar to bearings previously discussed, with the exception that the referenced downstream pressure of the gas side bushing can change. In order to ensure a constant flow across the ‘orifice’, the differential pressure must be kept constant. Therefore, every compressor seal system is designed to maintain a constant differential against the gas side seal. The means of obtaining this objective will be discussed as we proceed.
The other seal in the system is the atmospheric bushing, whose purpose is to minimize the flow of seal liquid to an amount that will remove frictional heat from the seal. This bushing can be conceptualized as a bearing, since the downstream pressure is usually atmospheric pressure. In the systems that directly feed into a bearing, the atmospheric bushing downstream pressure will be constant, at approximately 138kPa(20 psi). However the upstream supply pressure will vary with the pressure required by the sealing media in the compressor.
As an example if a seal system is designed to maintain a constant differential of 34.5 kPa (5psi) per square inch between the compressor process gas and the seal oil supply to the gas side bushing, the supply pressure with zero process gas pressure would be 34.5 kPa (5 psi)to both the gas side bushing and atmospheric bushing. Therefore, gas side bushing and atmospheric bushing differential would both be equal to 34.5kPa (5 psi). If the process gas pressure were increased to 138kPa (20 psi), the seal oil system would maintain a differential of 34.5 kPa ( 5 psi) across the gas side seal, and the supply pressure to the gas side bushing and atmospheric bushing would be 172 kPa(25 psi). In this case, the differential across the gas side bushing would remain constant at 34.5 kpa (5psi), but the atmospheric bushing differential pressure would increase from 34.5 to 172 kPa (5 to 25 psi). As a result, a primary concern in any seal liquid system is the assurance the atmospheric bushing receives proper fluid flow under all conditions. After the seal fluid exits the seal chamber, it essentially runs through two additional subsystems.
(To be continued)