To prevent a unit shutdown on low oil pressure, continuously vent the non-operating cooler and filter by installing orificed return lines, and keep the transfer valve bypass fill line opened to ensure warm-up of the cooler and filter prior to transfer, in cold climates. In cold climates (ambient temperatures below 15oC at any time of the year), cool, static oil in the non-operating cooler and filter will cause a transient pressure drop when it is charged on-line. This action can cause critical equipment unit trips on low oil pressure when the auxiliary pump did not start, or did not start in time.
Requiring orificed vents on all coolers and filters, and keeping the non-operating vent lines open as well as the cooler and filter till lines, will ensure that non-operating coolers and filters are always maintained at the same temperature as the operating vessels, and are vented and ready for a successful transfer. Where a trip has been caused by the issue noted above, operating procedures should be revised and orificed vents installed if required.
Design and function
The function of the transfer valves in the auxiliary system is to allow transfer from one bank of components (coolers, filters, etc.) to the stand-by bank of components without significant pressure pulsations being introduced into the system. In addition, transfer valves must be designed to positively shut off the unused components to allow for maintenance while the system is in operation.
Types of transfer valves vary widely. A six-port transfer valve allows flow into the valve to be diverted either to the left bank or right bank of components. Other types of valves include the standard, globe type valve. Both types exhibit the characteristic of minimal pressure change when transferring from one bank to the other. The exported valve should be designed with a lifting jack when a taper plug is used. This feature allows transfer of valves from one bank to another easily and still ensures tight shutoff of the valve when in its proper position.
Selection of material
Selection of the valves should include material and sizing considerations. Materials of construction should be carbon steel as a minimum with stainless steel internals. The use of bronze components should not be considered in systems that can incorporate gas entrained in the system fluid. In such systems brass, copper or bronze is not permitted. Valves are sized to match piping sizes in the system, which is normally designed for fluid velocities of four to six feet per second.
Reliability considerations
Transfer valves including lifting jacks are susceptible to valve plug damage. Frequently, operators do not employ lifting jacks when transferring valves. This type of valve can become bound and even break at the stem, necessitating critical equipment shutdown.
Prior to transferring flow from one bank to another, the bank to be transferred to must be full of auxiliary system fluid and properly vented. If it is not the capacity of coolers, filters and piping in the bank to be transferred to act as a vessel, and consequently reduce flow and pressure to the critical equipment, which will shut down the unit. Also, improperly vented components containing air or gases can cause control valve instabilities which can also lead to equipment shutdown.
Unit trips experienced during cooler and/or filter transfers are usually blamed on a ‘dead spot’ in the transfer valve when in fact, cool static oil of high viscosity in the non-operating cooler and/or filter caused the unit to trip on low oil pressure. It should be noted that all types of transfer valves (six-way, ball valve design, etc.) do not have a ‘dead spot’ that can cause an oil system pressure drop when transferred.
This best practice has been used since the mid-1990s, when unit trips were experienced that were caused by a cooler and filter transfer to cold, static oil. Since that time, all projects, with ambient temperatures below 15oC at any time of the year, have been required to have cooler and filter vents, and P&IDs indicating that valves were in open condition. All field auxiliary system audits also recommended this approach when ambient temperatures could fall below 15oC.
