LUBE OIL RUNDOWN TANK TANK – – SYSTEM DESIGN AND OPERATIONAL OPERATIONAL ASPECTS ASPECTS By:
13 JAN 2016
Muhammad Imran – Mechanical Mechanical Rotating Engineer – LUKOIL LUKOIL International Services B.V Dubai
F
orce feed lube oil system is used for large rotating equipment including heavy duty Pumps, Compressors and their drivers. Emphatically speaking, lube oil system is the life line for Rotating
Equipment and plays a pivotal role in achieving the plant reliability reliability and availability. availability. A badly
designed lube oil system directly impacts sustained operation of the the main equipment. History is replete with cases when a poorly designed lube oil system was the root cause of main m ain equipment damage. Bearings are sensitive machine elements; their life and performance heavily depend upon continuous supply of clean lubricant at a certain required Flowrate, pressure and temperature. A force feed lubrication system is used for this purpose which is a closed loop system consisting of a number of components for storing (Reservoir), pressurizing (Pumps), cooling (Heat Exchangers), filtering and maintaining a continuous supply and return of lubricant to each bearing through a network of piping, valves, fittings and supported by a necessary instrumentation and control system.
Figure-1: A Typical Force Feed Lube Oil System
Rotor-Bearings interface generates tremendous amount of frictional heat and consequently excessive rise in the bearing temperature. If not properly dissipated in a continuous manner, this frictional heat
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would cause burning and ultimate melting of the bearing metal. Figure-2 shows a typical shaft and bearing damage due to excessive temperature rise due loss of lubrication to t he bearing.
Figure-2: Typical shaft and bearing damage due to loss of lubrication
Frictional heat generation and temperature rise of the bearings is a function of several factors including static and dynamic loads, rotor speed and coefficient of friction etc. While static loads remain constant, the dynamic loads and rotor shaft speed do vary. The rotor shaft speed grows from zero to the operating speed during startup and decays from the operating speed to zero speed after shutdown. The time period in which the rotor speed decays from the operating speed until a complete stoppage is called coast-down period or run-down period. Figure-3 shows a typical coast-down curve for a 5.7 MW motor driven centrifugal compressor train.
Figure-3: Typical Coast-Down Curve for a 5.7 MW Motor Driven Centrifugal Compressor Train
Dynamic loads and rotor speed gradually decays during coast-down, however static loads and coefficient of friction remain constant. Therefore, rotor-bearing system continues to generate heat during coast2|Page
down, though at a decreasing rate, and hence bearings still require continuous supply of lubricant until the rotor comes to a zero speed. A well designed lube oil system should be able meet bearing lubrication requirements during normal operation as well as during the entire coast-down period. There are cases, for example power loss to the AC motor driven lube oil pumps, when both main and auxiliary lube oil pumps are not available causing total loss of lubrication. In this case main motor is also tripped (emergency shutdown by PALL) however the train bearing system still requires sustained lubrication for a safe shutdown to avoid bearing damage during the coast-down period. Various solutions are used for providing bearing lubrication during coast-down period in such emergency situation. This paper describes design and operational aspects for atmospheric (non-pressurized) Lube Oil Rundown Tanks which is one of the commonly used solutions for coast-down lubrication. LUBE OIL RUNDOWN – AN OVERVIEW:
Lube oil rundown tank is a Horizontal or Vertical storage vessel which is installed at a certain height above the shaft centerline of rotating equipment. The rundown tank holds a certain volume of lube oil during normal operation and provides gravity flow to the bearings after shutdown during coast-down. The tank elevation provides static head/pressure for the proper lube oil flow under gravity.
Figure-4: Schematic view of Compressor Train with Lube Oil Rundown Tank
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Figure-5: A 3.5 MW Centrifugal Compressor Train with Lube Oil Rundown Tank
Figure-6: Closer view of Lube Oil Rundown Tank in Figure-5 4|Page
RUNDOWN TANK SYSTEM COMPONENTS:
Figure-7 shows a simplified flow diagram for a typical force feed lube oil system with lube oil rundown tank and associated system components.
Figure-7: Simplified Flow Diagram for a Typical Force Feed Lube Oil System 5|Page
Following table summarizes rundown tank system components and their function relative to Figure-7.
Item
Description
Function
1
AT M Vent / Air breather
Venting and to break the vacuum inside rundown tank as the tank is emptied during coast down period.
2
Level Transmitter
To provide low level alarm and high level start permissive.
3
Rundown tank
To hold certain volume of lube oil during normal operation. To act as visible local indication to ensure that the rundown tank is filled up during filling and subsequently act as indication to ensure that lube oil is under continuous recirculation during normal operation.
Sight glass 4
5
6
7
8
Filling valve
NOTE-1
Check valve
NOTE-2
Bi-directional line
Orifice plate
NOTE-3
9
Check valve
10
Overflow / Recirculation line
NORMNALLY CLOSED and is (manually/automatically) opened to fill the rundown tank during each startup. To block main LO flow to rundown tank during normal operation. 1. To fill rundown tank during startup using lube oil pump. 2. To act as lube oil supply line from rundown tank to the lube oil header during coast-down period after shutdown. To provide a controlled leak path in the rundown tank system to ensure that during normal operation, the lube oil from the supply header is continuously re-circulated through the rundown tank back to the lube oil reservoir. To ensure lube oil rundown tank feeds lube only to the equipment; preventing any lube oil flow going directly back to the lube oil reservoir during coast down per iod. 1. To act as overflow line during initial filling of rundown tank. 2. To act as a recirculation line during normal operation.
NOTE-1: API 614 base requirement is manual valve, solenoid operated valve can be used as an option for automatic filling operation. NOTE-2: API 614 requires this check valve to be drilled. The drilled hole acts as a leak path to achieve lube oil recirculation through rundown tank during normal operation. NOTE-3: API 614 does not specify this orifice plate. However, some end users prefer this orifice plate as an alternative design. If this orifice plate is provided, then the check valve (item: 6) is provided as undrilled.
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RUNDOWN TANK SYSTEM – OPERATIONAL ASPECTS:
The rundown tank system operation can be described under three stages. Stage-1 – Start-up: Rundown tank is empty during first startup and after each emergency shutdown. Rundown tank level is used as one of the start permissives in the start logic of the main rotating equipment. Therefore, it is necessary to first fill the lube oil rundown tank. For this purpose, the Auxiliary Lube oil pump (AOP) is started and following lube oil flow is established in a closed loop.
LO Reservoir Auxiliary LO Pump LO Cooler & Filters LO Supply Header Equipment Bearings Drain Header LO Reservoir Then rundown tank filling valve (Item# 5/Figure-7) is opened and lube oil flow is established to the rundown tank. The sight glass (Item# 4/Figure-7) is watched by the field operator to ensure that rundown tank filling operation is successfully established. Once the tank is filled upto desired level, the Level Low Alarm (LAL) disappears, the High Level Alarm (LAH) is indicated and the start permissive for the rundown tank level NOT LOW is cleared in the control system. This completes the rundown tank filling operation. At this point, the rundown tank filling valve (Item# 5/Figure-7) is returned back to its normally closed position. See Figure-8 for illustration of this stage. Stage-2 – Normal Operation of Main Equipment: Main Lube Oil Pump (MOP) keeps the lube oil supply header pressurized and maintains lube oil flow to the bearings. Rundown tank filling valve is closed and lube oil flow to the rundown tank is blocked by the check valve (Item# 6/Figure-7) except a small quantity of flow passing through the orifice (Item# 8/Figure-7) or check valve (Item# 6/Figure-7) drilled hole if a drilled check valve is used instead of orifice plate. This small leakage flow through the o rifice plate or check valve drilled hole maintains a continuous lube oil recirculation back to the lube oil reservoir through the overflow line (Item# 5/Figure-7). This recirculation helps to avoid lube oil stagnation and sediment accumulation in the rundown tank. Also, it helps to maintain the rundown tank oil temperature at or closer to the lube oil supply temperature to the bearings.
See Figure-9 for illustration of this stage. Stage 3 – Coast-Down after ESD Trip: By virtue of the system design (i.e. rundown tank elevation), the maximum Static Head by rundown tank is lower than lube-oil trip set pressure. Therefore, static head in the lube oil rundown tank does not maintain oil flow to the lube oil supply header as long as the lube oil supply header pressure is maintained above the lube-oil trip set pressure.
When both the main and auxiliary lube oil pumps are lost such as in case of total power failure, the lube oil supply header pressure falls below the lube-oil trip set pressure. The main equipment trips and enters into its coast-down period. Soon after this moment, the Static Head from rundown tank is relatively higher than the prevailing LO header pressure. Therefore, a gravity flow is established by the rundown across the check valve (Item# 6 /Figure-7) down to the bearings through the LO supply header. See Figure-10 for illustration of this stage. 7|Page
STARTUP / RUNDOWN TANK FILLING
Figure-8: Lube Oil System Startup and Rundown Tank Filling 8|Page
NORMAL OPERATION
Figure-9: Lube Oil System and Rundown Tank – Oil flow during Normal Operation 9|Page
COAST-DOWN AFTER ESD TRIP
Figure-10: Lube Oil System and Rundown Tank – Oil flow during Coast-Down Period 10 | P a g e
RUNDOWN TANK SYSTEM - DESIGN ASPECTS:
The lube oil rundown tank system is designed as pe r minimum requirements in the API 614 standard and good engineering practices as descr ibed below. Rundown Tank Capacity: Rundown tank capacity is the most critical design parameter. The capacity sizing should be such that it provides lube oil gravity flow for the entire coast-down period ensuring safe shutdown of the equipment without any bearing damage. The tank size depends upon the required lube oil supply flowrate (m3/h) and total coast-down time. API 614 requires rundown tank to be sized for not less than 3 minutes (min. recommended coast-down time) of normal operating lube-oil flow. Tank Volume (L) = Normal Lube Oil Flow (L/min.) x Coast-Down Time (min.) Coast-Down Time: The actual coast-down time is provided by the supplier of the equipment to be served by the lube oil system. The coast-down time is affected by the train inertia as well as the system pressure (resistance). The lower the system resistance during the coast down period, the longer the equipment will continue to run before reaching zero speed. Therefore, calculation of coast-down time must include the process information such as coast-down is considered with or without depressurization. Similarly, the operating speed before trip also affect the coast-down period. The lower the speed before trip, the longer is the coast-down time. Therefore, for variable speed machines, the coast-down time should be evaluated for various operating speeds and longest coast-down time should be selected. Tank Maximum Static Head - Installation Elevation: The rundown tank is installed at a certain specific height above the equipment centerline. API 614 stipulates that the vendor shall specify the allowable minimum and maximum height of the rundown tank bottom nozzle above the machine centerline; the maximum Static Head shall not be less than the lube-oil trip pressure but not less than 30 kPa (0.3 barg; 5 psig) at the beginning of coast-down. This maximum static head consideration is important to ensure that there is no interference with the normal trip function based on PALL.
Figure-11: Schematic illustration for various Pressure Levels and T ank Maximum Static Head Note: the above schematic is just conceptual illustration and does not represent actual pressure profile.
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Lube Oil Recirculation: API 614 specifies a drilled check valve (Item#6 / figure-7) in the rundown tank system. The purpose of drilled hole is to facilitate a continuous leakage flow to the rundown tank during normal operation. This leakage slowly but continuously raises the rundown tank level and excess oil flows back to the lube oil reservoir via overflow line. This design ensures that there is no stagnation and hence no accumulation of sediments in the rundown tank. Also, this recirculation maintains the lube oil temperature inside the rundown tank at or closer to the lube oil temperature in the lube oil supply header. This prevents feeding of a cold/viscous lubricant to the bearings during Coast-down event and avoids bearing damage due to thermal shock. In the alternate design approach, an orifice plate (Item# 8/Figure -7) is provided instead of a drilled check valve for the recirculation flow. If this orifice plate is provided, then check valve (Item#6 / figure-7) is provided as undrilled. The orifice size or the drilled hole size should be designed such that the whole contents of the rundown tank are renewed with fresh lube oil in a couple of hours’ time. Sight Flow Glass in the Overflow Line: API 614 specifies a sight flow glass in the overflow line. This sight glass acts as visible local indication to ensure that the rundown tank is filled up during filling and subsequently act as indication to ensure that lube oil is under continuous recirculation during normal operation. This sight flow glass should be located such that it can be readily observed such as the oil reservoir o r equipment operating deck. Rundown Tank Level Monitoring: API 614 specifies that rundown tank shall be provided with low-level alarm and high-level permissive start functions. API 614 figure B.15 shows two level switches LSL and LSH for these functions. However, API 614 also accepts that these functions may be served by a common device. Therefore, instead of two level switches, a single level transmitter c an also be used for LAL and LAH functions. Rundown Tank Filling Valve: API 614 specifies a manually operated filling valve as base scope. This valve is normally closed and opened only during rundown tank filling before the equipment startup. However, this valve may also be solenoid-controlled to allow an automated start. Rundown Tank Material: A careful consideration to the rundown tank material is extremely important. The rundown tank represents a large vessel which is connected to the lube oil system downstream of lube oil filters. Therefore, any rust, scale or corrosion in the rundown tank will go directly into the bearings during coast-down lubrication and could cause damage to the bearings. API 614 specifies that lube oil rundown tank shall be made of austenitic stainless steel mater ial. Rundown Tank Drain Connection: As one of best practice, the rundown tank drain nozzle is extended (usually 25 mm) inside the tank. This design helps to retain any foreign matter/sediments in the tank bottom and allow only the clean oil to flow down the drain nozzle from top of nozzle projection. In this case, sometimes an additional flush mounted drain connection is provided for complete drainage during maintenance. See typical design shown in Figure-12.
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Rundown Tank Provision for Inspection: API 614 specifies a DN 150 (NPS6) nozzle for accessing and inspecting the interior of the rundown tank shall be provided. Alternatively, a bolted cover can be provided in lieu of the DN 150 (NPS6) nozzle for access and inspection. See typical design shown in Figure-12 .
Bolted cover for Ins ection
Projected drain nozzle
Inspection Hole
Projected
Additional drain
drain nozzle
nozzle flush mounted Figure-12-a: RDT with Inspection Cover
Figure-12-b: RDT with Inspection Hole
CONCLUDING REMARKS:
Lube oil unit is the most critical auxiliary system for rotating machinery. The operational and design aspects of each system component should be fully known and understood by all the concerned engineers. Large rotating equipment involves machinery trains with high inertia loads and considerable coast-down time after shutdown. Bearing lubrication during coast-down period is critical in case of emergency shutdown situation when the AC motor driven lube oil pumps are not available. Lube oil rundown tanks are commonly used as a most reliable solution for coast-down period lubrication during emergency shutdown.
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Rundown tank capacity is the most important design element. There are cases when inadequate capacity of rundown tank caused equipment damage during coast-down. Therefore, a close coordination should be done with the rotating equipment vendor to ensure that the all the design parameters are properly considered for adequate sizing of the rundown tank. Actual coast-down time calculation/curves should be reviewed to ensure that all the relevant design inputs have been considered in the calculations. The design basis for the recirculation orifice or check valve drilled hole should be clarified to ensure that a proper recirculation flowrate has been considered. The recirculation flowrate should be able to achieve total renewal of the rundown tank inventory in a couple of hours’ time during normal operation. Rundown tank installation height should be clarified with machinery vendor and the information should be transferred to the relevant engineering disciplines. The tank installation height should be verified during the 3D model review and it should be included in the checklist to be verified after mechanical completion to ensure that vendor recommendation for tank installation height has been considered. When rundown tank is used in the system, the lube oil reservoir sizing must be done such that it is able to receive rundown tank total inventory volume without causing overflow after the coast-down period. The check valve (Item# 9/Figure-7) is critically important and should be included in the check list after mechanical completion to ensure that the check valve is present in the system. In addition, make sure that the tie-in for rundown tank fill/drain line is connected to the lube oil header downstream of this check valve. Failing to meet these requirements will make the rundown tank system in-effective due to loss of lube oil back to the r eservoir without reaching the bearings.
About the author
is Mechanical Rotating Equipment Engineer presently working with LUKOIL International Services B.V based in Dubai. He has 15 years of professional experience in Oil & Gas and Petrochemical projects including EPC detail engineering, PRE-FEED, FEED and hands-on experience in the Pre-commissioning, Commissioning, Startup and initial operations. Before joining LUKOIL, he worked for WorleyParsons Qatar as Lead Mechanical Design Engineer for Rotating Equipment and Packages. He can be reached at “
[email protected]”. Muhammad Imran
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