Page 474 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Lube, Seal and Control Oil System Best Practices Best Practice 7 .34
the seal oil system would maintain a differential of 34.5 kPa (5 The supply system
PSI) across the gas side seal, and the supply pressure to the gas
side bushing and atmospheric bushing would be 172 kPA (25 Referring to definition of a seal oil system, it can be seen that its
PSI). In this case, the differential across the gas side bushing function is identical to that of a lube oil system, with one
would remain constant at 34.5 kPA (5 PSI), but the atmospheric exception. The exception is that the seal fluid must be delivered
bushing differential pressure would increase from 34.5 to 172 to the seals at the specified differential pressure. Let’s examine
kPA (5 to 25 PSI). As a result, a primary concern in any seal this requirement further.
liquid system is the assurance that the atmospheric bushing Refer again to Figure 7.34.2. Notice that the atmospheric
receives proper fluid flow under all conditions. After the seal bushing downstream pressure is constant (atmospheric pres-
fluid exits the seal chamber, it essentially returns through two sure). However, the gas side bushing pressure is referenced to
additional subsystems. the compressor process pressure. This pressure can and will
vary during operation. If it were always constant, the re-
C The atmospheric draining system quirement for differential pressure control would not be
present in a seal system and would be identical to that of a
The flow from the atmospheric bushing, if it does not lubricating system. Another way of visualizing the systems is to
directly enter the bearing system, will return to the seal oil understand that the lube system utilizes differential pressure
reservoir. In addition, flow from any downstream control valve control as well, but the reference pressure (atmospheric pres-
will also return through the atmospheric drain system to the seal sure) is constant and consequently all control valves need only
oil reservoir. Both these streams should be gas free since they control lube oil pressure. However seal systems require some
should not come in contact with the process gas. means of constant differential pressure control (reference gas
pressure to seal oil supply pressure). This objective can be
accomplished in many different ways. Referring back to
D The seal leakage system Figure 7.84 it can be seen that the supply system function is
identical to that of a lube oil system with the exception that
The fluid that enters the gas side bushing is controlled to the liquid is referenced to a pressure that can vary and must be
a minimum amount such that it can be either discarded or controlled to maintain a constant differential between the
properly returned to the reservoir after it is degassed. Typically, referenced pressure and the seal system supply pressure.
this amount is limited to less than 77 liters (20 gallons) per day The sizing of the seal oil system components is also identical to
per seal. Since this liquid is in contact with the high speed shaft that of the lube oil system components. Refer back and observe
it is atomized and combines with sealing gas to enter the leakage the heat load and flow required of each seal is determined in
system. This system consists of: a similar way to that of the bearings. Seals are tested at various
speeds and a necessary flow is determined to remove the heat
- An automatic drainer of friction under various conditions. The seal oil flow re-
- A vent system quirements and corresponding heat loads, are then tabulated
- Degassing tank (if furnished)
and pumps exchangers, filters, and control valves are sized
The function of each component is as follows: accordingly.
The seal oil reservoir is sized in exactly the same way as
The drainer
The drainer contains the oil-gas mixture from the gas side lube oil reservoir in our previous example. The only major
seal. The liquid level under pressure in the drainer is controlled difference between the component sizing of a seal and a
by an internal float or external level control valve to drain oil bearing is that the seal flows across the atmospheric bushing
back to the reservoir or the de-gassing tank, as required. will change with differential pressures. Every liquid compressor
seal incorporates a double seal arrangement. The gas side
The vent system
The function of the seal oil drainer vent system is to seal differential is held constant by system design. The atmo-
ensure that all gas side seal oil leakage is directed to the spheric side seal differential varies with varying seal reference
drainer. This is accomplished by referencing the drainer vent (process) pressure. Therefore, the total flow to the seals will
to a lower pressure than the pressure present at the gas side vary with process pressure and must be specified for maximum
seal in the compressor. The drainer vent can be routed back and minimum values when sizing seal system components.
to the compressor suction, suction vessel or a lower pressure Remembering the concept of an equivalent orifice, a compressor
source. at atmospheric conditions will require significantly less seal oil
flow than it will at high pressure 1,380 kPa (200 PSI) condi-
The degassing tank
This vessel is usually a heated tank, with ample residence tions. This is true since the differential across the atmospheric
time (72 hours or greater) to sufficiently de-gas all seal oil such seal and liquid flow will increase from a low value to a signifi-
that it will be returned to the reservoir and meet the seal oil cantly higher value, while the gas side bushing differential and
specification (viscosity, flashpoint, dissolved gasses, etc.). These liquid flow will remain constant provided seal clearances remain
items will be discussed in detail later. constant.
We will now proceed to discuss each of the major sub- Many seal system problems have been related to insufficient
systems in detail, defining the function of each such that the seal oil flow through the atmospheric bushing at low suction
total operation of a seal system can be simplified. pressure conditions. Close attention to the atmospheric drain

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