Page 421 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 7 .13 Lube, Seal and Control Oil System Best Practices
A two way valve is a simple bypass around the cooler, while Critical equipment trip instrumentation
a three way valve is a true mixing valve. It should be noted that A general critical equipment design philosophy is to avoid trip
when sizing a two way valve, the pressure drop across the cooler
circuits as much as possible. That is, to only install trip switches
must be known, to ensure that the valve coefficient is large in those situations which are absolutely necessary. Typically,
enough to pass the required flow. Many systems using two way
valves are insufficiently sized. This can result in cooler oil con- auxiliary systems incorporate only one trip function, such as the
low lubricating oil trip in lube systems and the low seal oil dif-
stantly being supplied to the system, since the pressure drop ferentials trip in seal systems. Sometimes a high temperature
through the cooler is less than the minimum pressure drop switch is also installed to trip, but this is not usually the case.
through the control valve.
The setting of the trip switch is very important. It must be se-
lected such that the equipment will shut down when actuated in
Instrumentation order to prevent any long term damage. It must also be selected
to prevent spurious, unnecessary trips of the unit, since they are
The instrumentation in any auxiliary system is extremely extremely costly in loss of revenue. In addition, the quality of
important in ensuring quick system response, accurate moni- any trip switch is extremely important since this relatively low
toring of system condition and rapid system shutdown in the cost device could cost millions of dollars of lost product revenue
event of upsets. In this section we will examine stand-by per day if it malfunctions. Attention is drawn to correct selec-
pump start-up operation, critical equipment shutdown and tion of switch component materials to prevent corrosion or any
monitoring functions of the instrumentation in the auxiliary abnormality that would cause drifting of switch setting and
system. unnecessary unit shutdowns. Again the concept of a system is
extremely important to consider. It must be remembered that
the trip switch and the shutdown system for the critical
Stand-by pump automatic start equipment must function accurately together in order to
As previously mentioned, interrupting pumped flow to critical terminate equipment operation immediately upon signal from
equipment results in a rapid deterioration in system flow and the initiating trip switch. ‘Best practice’ is to use smart
pressure. Referring again to the concept of an equivalent vessel, (self-diagnostic), triple redundant (two out of three voting)
the absence of inlet flow to the system while exit flow is con- transmitters for all pump start and trip services.
tinuing will instantaneously produce a pressure drop. This con-
cept is used in a pressure switch that signals the immediate start Auxiliary system monitoring
of the stand-by pump. Practice has shown that locating the Refer to Figure 7.13.1 and observe the different monitoring and
pressure switch takeoff as close as possible to the pump dis- alarm functions normally used in an auxiliary system. In order to
charge results in the quickest response time to start the stand-by ensure reliable auxiliary system operation, the personnel must
pump. Some systems incorporate dual pressure switches, one continuously observe and record any changes in instrument
close to the pump discharge and another up close to the critical readings and promptly attend to alarms to ensure that the
equipment. Both switches start the stand-by pump on signal. system continues to operate as required. Changes in any of the
The pressure setting of the switch is usually set just below the system instruments indicates a change in the operating condition
lowest discharge pressure that the pump will produce. In order of the system, which must be followed through to make sure
to ensure the rapid start of the auxiliary pump when required, that components are operating as required. As an example,
many systems incorporate an on-off-automatic switch on the slowly deteriorating lube oil supply pressure could indicate
auxiliary pump, or on the stand-by pump motor starter for either a valve malfunction, reduction in speed of a main turbine
testing the system. It is extremely important that the position of pump driver, excessive pressure drop in the system oil filter or
the switch always be in the automatic mode during critical several other types of problem. It is extremely important to
equipment operation. It is recommended that an alarm be maintain a program of auxiliary system instrumentation cali-
supplied and annunciated in the event that the auxiliary or bration to ensure all instruments are reading properly. This will
stand-by pump is not in the auto position during critical aid greatly in determining system malfunctions and assist in the
equipment operation. site preventive maintenance program.

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