Page 385 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer
P. 385

Be st Practice 7 .5           Lube, Seal and Control Oil System Best Practices
                                                            i.e. values approaching 220 centistokes (1,000 SSU) e on the
                                                                    1
                                                            order of 1 / 2 to 2 times the selected valve coefficient without
                                                            viscosity considerations.
                                                            Control valve sensing line snubber devices
                                                            (dampers)
                                                            If these devices are included, a review of device design and
                                                            confirmation of proper installation should be confirmed. Such
                                                            devices provide unrestricted flow in one direction and restricted
                                                            flow in another. The total auxiliary system operation must
                                                            be reviewed in this light to confirm proper installation and
                                                            orientation.

                                                            Supply pipe velocity checks
                                                            The pump header, interconnecting console pipe, and piping
       Fig 7.5.5   Control valve flow characteristics (Courtesy of Fisher Controls,  to the unit should all be checked for proper fluid velocity.
       Inc.)
                                                            Typical velocity values in auxiliary system supply pipes are
                                                            on the order of four to six feet per second velocity. Velocity
                                                            is derived from the following equation for incompressible
          The maximum flow for this condition would be the normal  flow:
          bypass flow of the main pump plus the total flow of the
          auxiliary pump.                                                    Qft=min ¼ A   V
                                                                                               2
                                                                                          2
                                                            Where: A ¼ Internal pipe area (m or ft )
          Attention is drawn to the characteristic of the valve curve for  V ¼ Fluid velocity (m/sec or ft/sec)
       this application. The normal operating point would be approx-  Charts for standard pipe sizes and schedules are available to
       imately the minimum C v , therefore a valve characteristic that  determine velocities (see Figure 7.5.6).
       results in a fairly significant (15e25%) valve travel for this small  Note that schedule 80 is usually used for carbon steel pipe
       C v would be desired (quick opening). Two pump operation  below 2". Schedule 40 is used above 2". For stainless steel pipe,
       (maximum C v ) is an abnormal case. Therefore the valve should  schedules 10 and 20 are used respectively.
       be designed merely to pass this flow (refer to Figure 7.5.5)at                                    1   1
                                                              Typical drain line velocities are 0.15e0.08 m/sec ( / 2 to / 4
       90% or less of the valve catalogue C v .
                                                            feet/sec). Attention is drawn to the need to properly size drain
                                                            pipes for installations, where critical equipment is significantly
       Pressure reducing valve                              elevated above the reservoir. All drain pipes should be sized with
                                                            adequate area, to preclude excessive air being entrained with the
       In a centrifugal pump application, pressure reducing valves
       would experience minimum, normal and maximum C v s similar  oil to promote drainage back to the reservoir. An additional
                                                            consideration for supply headers at the unit is that supply
       to bypass valves, with the exception that downstream valve
       pressure will change with increasing flow.            headers are frequently sized for one standard pipe dimension. In
                                                            the case of large, critical, equipment units (two or three bodies
          When pressure reducing valves are used to reduce pressure
       levels (control oil pressure to lube oil pressure, seal oil pressure  and driver), the amount of oil from the entrance to the header to
                                                            the last component decreases significantly. In an effort to min-
       to lube oil pressure, etc.) the valve C v should be selected for all
       possible operating cases, as mentioned above for bypass valves.  imize pipe size, many vendors specify small size headers, so
                                                            pressure drop here is excessive and requires a higher supply
       Care should be taken to ensure all possible operating cases are
       considered.                                          header pressure than anticipated in the unit design. Improper
                                                            sizing of critical equipment supply headers could cause exces-
                                                            sive flow across equivalent orifices (bearings), thereby requiring
       Temperature control valves                           all flow of the main pump and necessitating the operation of the
                                                            auxiliary pump.
       The temperature control valve C v will remain relatively constant
       under all auxiliary system conditions. Two way valves, however,
       must be sized such that the full flow pressure drop across the  Transfer valve sizing
       valve is less than the clean pressure drop across the cooler in
                                                            Transfer valve configuration and materials of construction
       parallel with this valve.                            should be confirmed at this point. Transfer valve design should
          Differential pressure control valves and level control valves  be checked to confirm tight shutoff.
       are sized and examined in the manner described above for
       bypass and pressure reducing valves. Details will be discussed
       in subsequent chapters. Viscosity corrections are required for  Cooler sizing
       all control valve sizing when operating viscosities exceed 7.4  The cooler data sheet should be reviewed to confirm correct
       centistokes (50 Sabolt Universal Seconds [50 SSU]). Signifi-  duty, cooling media details, fouling factors and materials of
       cant size increases are required for high viscosity operation e  construction.

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