Page 216 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 3 .22 Compressor Best Practices
D. The seal leakage system present in a seal system and it would be identical to a lubricating
system. Another way of visualizing the system is to understand
The fluid that enters the gas side bushing is controlled to that the lube system utilizes differential pressure control as well,
a minimum amount, such that it can be either discarded or but the reference pressure (atmospheric pressure) is constant,
properly returned to the reservoir after it is degassed. Typically, and consequently all control valves need only control lube oil
this amount is limited to less than 77 liters (20 gallons) per day, pressure. However seal systems require some means of constant
per seal. Since this liquid is in contact with the high speed shaft differential pressure control (reference gas pressure to seal oil
it is atomized and combines with sealing gas to enter the leakage supply pressure). This objective can be accomplished in many
system. This system consists of: different ways. Referring back to Figure 3.22.1 it can be seen
that the supply system function is identical to that of a lube oil
- An automatic drainer system with the exception that the liquid is referenced to
- A vent system a pressure that can vary, and must be controlled to maintain
- Degassing tank (if furnished) a constant differential between the referenced pressure and the
seal system supply pressure. The sizing of the seal oil system
The function of each component is as follows:
components is also identical to that of the lube oil system
components. Refer back and observe that the heat load and flow
The drainer required of each seal is determined in a similar way to that of the
bearings. Seals are tested at various speeds, and a necessary flow
The drainer contains the oil-gas mixture from the gas side seal. is determined to remove the heat of friction under various
The liquid level under pressure in the drainer is controlled by an conditions. The seal oil flow requirements and corresponding
internal float, or external level control valve, to drain oil back to heat loads are then tabulated, and pumps exchangers, filters, and
the reservoir or the de-gassing tank, as required. control valves are sized accordingly.
The seal oil reservoir is sized exactly the same way as the lube
oil reservoir in our previous example. The only major difference
The vent system between the component sizing of a seal and a bearing is that the
seal flows across the atmospheric bushing will change with
The function of the seal oil drainer vent system is to ensure that differential pressures. As previously explained, any liquid
all gas side seal oil leakage is directed to the drainer. This is compressor seal incorporates a double seal arrangement. The gas
accomplished by referencing the drainer vent to a lower pres- side seal differential is held constant by system design. The
sure than the pressure present at the gas side seal in the com- atmospheric side seal differential varies with varying seal ref-
pressor. The drainer vent can be routed back to the compressor erence (process) pressure. Therefore, the total flow to the seals
suction, suction vessel or a lower pressure source. will vary with process pressure and must be specified for max-
imum and minimum values when sizing seal system compo-
nents. Remembering the concept of an equivalent orifice,
The degassing tank a compressor at atmospheric conditions will require significantly
less seal oil flow than it will at high pressure 1,380 kPa (200 psi)
This vessel is usually a heated tank, with ample residence time conditions. This is true since the differential across the atmo-
(72 hours or greater) to sufficiently de-gas all seal oil, such that it spheric seal and liquid flow will increase from a low value to
will be returned to the reservoir and meet the seal oil specifi- a significantly higher value, while the gas side bushing differ-
cation (viscosity, flashpoint, dissolved gasses, etc.). These items ential and liquid flow will remain constant provided seal clear-
will be discussed in detail later. ances remain constant.
We will now proceed to discuss each of the major sub-sys- Many seal system problems have been related to insufficient
tems in detail, defining the function of each such that the total seal oil flow through the atmospheric bushing at low suction
operation of a seal system can be simplified. pressure conditions. Close attention to the atmospheric drain
cavity temperature is recommended during any off design (low
suction pressure condition) operation.
The supply system
Referring to definition of a seal oil system, it can be seen that its The seal housing system
function is identical to that of a lube oil system, with one ex-
ception. This is that the seal fluid must be delivered to the seals Regardless of the type, the purpose of any seal is to contain the
at the specified differential pressure. Let’s examine this re- fluid in the prescribed vessel (pump, compressor, turbine, etc.).
quirement further. Types and designs of seals vary widely. Figure 3.22.3 shows
Refer again to Figure 3.22.2 which shows an equivalent ori- a typical mechanical seal used for a pump.
fice diagram for a typical compressor shaft seal. Notice that the Since the contained fluid is a liquid, this seal utilizes that fluid
atmospheric bushing downstream pressure is constant (atmo- to remove the frictional heat of the seal and vaporize the liquid,
spheric pressure). However, the gas side bushing pressure is thus attaining a perceived perfect seal. A small amount of va-
referenced to the compressor process pressure. This pressure porized liquid constantly exits the pump across the seal face. It
can and will vary during operation. If it were always constant, is a fact that all seals leak. This is the major reason that many
the requirement for differential pressure control would not be pump applications today are required to utilize seal-less pumps

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