FAT: Twin Screw Multiphase Pumps, Subodh K Tikadar, CEng
Theory:
From oil well it is always required to take out fluid which is a mixture of oil, water and gas and in such application twin screw pump is a challenge. Twin screw pumps were mainly used for transferring viscous liquids and also gaseous hydrocarbon, e.g. propane, butane, LPG. Twin screw pump is good performer over a wide range of fluid viscosity and gas void fraction (GVF= ratio of ingested gas volume flow Q g,o to Total inlet volume flow, Qin) from 0% to 100%. Water, oil, gas mixtures or slug of 100 % gas can all be pumped at constant pressure with the screw pump. A very significant advantage of this pump is that its ability to maintain full discharge pressure handling all these fluids without the requirement for changing its operating speed. The variation of discharge flow Q of the pump with its pressure differential Δ p is one of the pump’s major characteristics.
The characteristics of twin screw pumps for high viscus fluids are strait line because the slip or leakage is laminar. But for low viscosity liquids, the strait line characteristics are not obtained. So low viscosity liquids are mostly transferred by centrifugal pumps. The slip or leakage rate is often turbulent and tends to vary as the square root of Δp, yielding a Q- Δp a concave curve downwards.
It is found that screw pumps possess the ability to handle a wide range of single and multiphase fluids upto 100% gas at any pressure rise from zero to the maximum design value.
Since this is a positive displacement pump the input shaft power Ps = Qd Δp + Pf ; Qd = displaced volume flow rate, Δp = differential pressure, Pf = Fluid friction drag of the rotors. The term Qd Δp is not affected by GVF, as would be the case of centrifugal machine where the Δp depends on the average fluid density and the rotor tip speed.
For the screw pumps, the test results in the plot of as X axis: Δp in PSI and Y axis- Intake volume flow rate Qin in USGPM, we see input shaft power Ps vs pressure rise Δp; this increasingly upwards rising curve shifts by a very small amount with GVF. The small difference is due to the change in the fluid friction drag power Pf across the rotor screws. So, a screw pump can ingest slugs of gas, incurring neither the large spikes in shaft torque nor the need for a sudden increase in rotative speed in order to maintain Δp. Hence we don’t feel to use gear box, i.e. speed varying devices are never supplied with screw pumps in order to accommodate the longer term, more permanent changes in total intake volume flow, liquid viscosity, etc., that accompany the changes in oil well production rates.
Multiphase Performance- Air/ Water Mixture:
The screw pump performance handling multiphase fluids is more complicated, and the shape of the curve can vary from concave upward to almost linear with little slip. The performance characteristics depends on many factors including rotating speed, differential pressure, GVF, liquid viscosity, and screw clearances. Lower GVF values result in less gas volume to compress, and the slip through the clearances is therefore less than with pure liquid but greater than with the higher GVF fluids. With increasing GVF, the characteristics become flatter. The effects of GVF, liquid viscosity, specific heat, pressure ratio and pressure rise, pump rotational speed, size, screw geometry, constant and variable internal clearances have been modeled and coded into program by manufacturer to suit it for a particular application.
A change in the operating conditions in the oil field that produces a lower liquid viscosity is a serious development that can significantly reduce the capacity of the twin screw pump. Lowering the liquid viscosity seriously impairs the performance of a twin screw pump whereas the same change enhances the performance of a centrifugal pump. If these two designs could be used interchangeably, the problem of low liquid viscosity could be overcome by changing the pump type.
Description of Multi Phase Pump (MPP) 2x 50% (2W) Parallel Operation:
Twin screw MPP is manufactured per API 676;
Coupling per API 671;
Mechanical seal oil system per API 682;
Lube oil system per API 614;
Electrical drive system including Motor, VSD/ VFD, Transformer;
Instrumentation & Control system;
Basic process Parameters (MPP):
Tag Nos: 1-01-00-U-0901/902;
2x50% of Multiphase pump operating in parallel shall be considered to meet the overall production. Say,
Capacity M3/H (Min/ Nor/ Rated / Max): 103.1 / 508.5 / 702.7 / 702.8
Suction Pressure Bar G Rated/ Max: 17.23 / 34.5
Discharge Pressure Bar G Min/Rated: 58.95 / 99.97
ΔP Bar Min/ Rated: 41.72 / 82.74 say 83
GVF Rated: 64%
Objective of FAT:
To ensure whether the MPP Skid package are able to provide all the process parameters required for the project at different conditions. And also to ensure proper and efficient functioning of the MPP Package.
Unit A: The complete MPP Skid Unit A is comprised of the following equipment, auxiliaries, and accessories:
a) Pump A
b) Job motor A
c) Job VFD A, Transformer A
d) Job Lube Oil System (LOS A)
e) Job Lube Oil System Cooler A
f) Job Barrier Seal Oil System (BSOS A)
g) Separator A
h) Interconnecting Piping between pump , motor, VFD, transformer, coolers, unit control panel (UCP A)
Unit B: The complete MPP Skid Unit B is comprised of the following equipment, auxiliaries and accessories:
i) Pump B
j) Job motor B
k) Job VFD B, Transformer B
l) Job Lube Oil System (LOS B)
m) Job Lube Oil System Cooler B
n) Job Barrier Seal Oil System (BSOS B)
o) Separator B
p) Interconnecting Piping between pump , motor, VFD, transformer, coolers, unit control panel (UCP B)
FAT shall be conducted on;
1. Spare Rotor
2. Unit A
3. Unit B
FAT for Spare Rotor:
Test medium: 100% pure city water with viscosity 1mm2/s, 0% GVF. Driver used is Test field motor.
a) Performance Test: Test readings shall be carried out @ rated speed, i.e., 100% speed, @ 60 % of rated speed and @ 90% of rated speed and min speed of 710 rpm considering all operating process parameters including guarantee point (Rated). The following operating points shall be measured and recorded;
- Suction pressure
- Discharge pressure
- Capacity/ Flow
- Motor power via VFD and UCP
- Temperature of the testing medium
- Speed
Test data shall be converted to the guaranteed conditions by means of the equations provided in the FAT Procedure furnished by MPP Vendor (ITTB).
The recorded readings shall be plotted and thus Characteristics curves plotted.
b) Mechanical Run Test: 4 H, No load condition (Throttle vales at suction and discharge sides open in Closed circuit test bed). Bearing Temperature and Vibrations are measured at pump Suction pressure, Discharge pressure at rated speed. The oil bearing temperature shall not exceed 100 OC
c) Strip Test: After finishing Mechanical Run Test of spare rotor, the pump is stripped and bearing housings are dismantled. The bearings and seals are visually inspected for any scratch, wear, scuff, etc.
FAT for Unit A:
The pump A, job motor A with VFD A, job coupling A, job LOS A and Cooler A, job BSOS A, Separator A, process interconnecting piping with valves between skids, job piping and job separator skid and Control unit, instrumentations and the required auxiliaries and accessories including electrical are connected to make a complete functional system for Unit A. Following tests shall be conducted step by step to ensure complete MPP package control system including field equipment. The test shall be carried out as per FAT check list:
Visual Inspection: Before start the FAT, complete unit shall be checked for correct connection between components, visible damage or any kind of imparity shall be noted and corrected. The same shall be followed after the FAT also.
a) Performance Test: Performance test is a part of FAT conducted to ensure the ability of the pump to provide capacity, differential pressure and power as per project datasheet and specifications.
Test medium is 100% pure city water with viscosity 1mm2/s, 0% GVF (Air intake shall be closed in the test skid set up). FAT @ rated speed (100% speed) according to the provisions stipulated in API 676 3rd Edition. Measuring points shall be selected between 0 barg and Rated pressure, say, @ Δp of 10, 20, 30, 50, 60, 70, 83 bar.
Also the following parameters shall be measured and recorded @ variable speed, say, 60% and 90% of rated speed & minimum speed 710 rpm and shall be used to draw pump characteristic curves:
Say,
- Suction pressure 17 barg
- Discharge pressure 100 barg
- Rated Δp 83 bar (Guarantee point )
- Capacity (min/nor/rated/max, m3/ h) 103.1/508.5/702.7/702.8
- Motor power 3.3 MW @ 1598 RPM
- Temperature of the testing medium, say, 60 0C
- Speed 1598 rpm
Test data shall be converted to the guaranteed conditions by means of the equations provided in the FAT Procedure furnished by MPP Vendor (ITTB).
Tolerances shall be per the following.
% of Rated Speed
+/- Tolerance(%) of Rated Capacity
Measured Capacity
100
3/0
90
3/0
60
3/0
Min speed, say, 710
3/0
Rated Power@ Rated pressure and Capacity
4/-
No negative tolerance is accepted.
To ensure necessary cooling of the test medium a suitable external cooler shall be used.
Since performance test medium is 100% water and no gas (0% GVF) and its Viscosity is not same as the real fluid, so the measured test data of Capacity and Drive power shall be converted to guaranteed Capacity and Drive power as per Manufacturer’s equations.
b) Full Load String Test: After performance test of the Unit A, its Full load string test @ rated condition shall be conducted to prove that the complete MPP system is satisfactorily working together. The load shall be increasing by closing the throttle valves. Test medium is 100% city water of viscosity 1mm2/s and 0% GVF (Air supple valve shall be closed). Suction pressure, rated discharge pressure and Δp are recorded only and match with the previous recorded ones and datasheet.
c) Mechanical Run Test: After Full load string test Mechanical run test shall be conducted with the same test fluid, i.e. 100% city water of viscosity 1mm2/s and 0% GVF (Air supple valve shall be closed in the test skid set up). It will be a 4 hours No load test, i.e. Throttle valves shall be remaining open (i.e., recorded Differential pressure Δp will be zero). In this test Bearing temperature and the Vibration shall be measured at 30 minutes interval and shall be recorded. Job temperature and vibration probes shall be used. The test duration is 4 hours for the purpose of stabilization of temperature and vibration. Criteria for temperature stabilization is 1 o C per 15 minutes average value over all the bearings, say, 4 bearings. The bearing oil temperature limits shall not exceed the preset alarm threshold for bearing temperature and the Lube oil circulation line temperature indicated in Alarm Trip Set point List.
Vibration shall be measured and recorded at certain predetermined interval, say, 30 minutes at the no load condition. There are two non-contacting horizontal probes (X, Y) and one Radial non-contacting probe (Z) at each bearing casing for vibration measurement. The vibration intensity (RMS) i.e the effective value of the vibration velocity (mm/s) is measured. The vibration limit is 10 mm/s RMS in full open throttle valve. The measuring performance shall be as per the relevant code, say, DIN ISO 10816, VDI 3836.
d) Dry Run Test/ Gas Slug Test: The air intake valve is fully remain open in the test set up. Duration of dry run test shall be 45 minutes. Medium is reverted from Separator for maintaining liquid film between the screws. The pump will start at zero suction pressure and 6 barg discharge pressure with water and air (GVF=30%). Throttle valve at pump discharge will be set manually to achieve the required back pressure. The required amount of water at the pump inlet will be calculated according to the displaced volume at the actual pump shaft speed. The volume flow of water will be set by the second Throttle valve installed between Water tank and Separator and will be measured by a Flow meter. Gas slug will be simulated by closing throttle valve installed in between Separator and Liquid tank. Now, the Throttle valve between Liquid tank and Separator is fully closed and keeping the pump Inlet flange fully open ( which is connected with Air intake valve) for the pump to operate with zero inlet pressure, 6 bar Δp, average gas content 100%, operating speed, max temperature at timing gear/ bearing cover 100 oC for 45 minutes. The oil bearing temperature limit shall not exceed 100 OC. During the testing, the temperature is checked at outside surface of bearing cover with contact thermometer and recorded every 5 minutes. Tightness of pump will be checked during this test.
e) Noise Test: There is no separate noise test. It is performed during Performance test and Full load string test. Noise measuring will be carried out at rated conditions, i.e., at inlet pressure, rated Δp 83 Bar and rated speed 1598 rpm. This test shall be conducted by TUEV NORD Umweltschutz to determine and assess the noise emissions. The linear- and A –rated sound [pressure level will be measured at every measurement point. At the same time the sound pressure level Lp [dB(A)] will measured for the octave band frequencies of 63 Hz up to 8000 Hz. Afterwards the measured A –rated sound pressure level Lp will be converted in Sound power level Lw. At measurement points where results could be obtained, the sound intensity was measured at a distance of 1 m at discrete points. The results of the measurements display the guaranteed value, say, 92 dB(A) for the skid. The noise measurements of the pumps shall be conducted at the operating points with reference to the Code EN 12 639. In the case of significant influence of ambient noise, measurement sound intensity according to EN ISO 9614-1 shall also be permissible.
The LOS and SOS’s Functionality tests had been performed in Sub-Vendors factory before FAT. No need to perform it while FAT.
FAT for Unit B:
The pump B, job motor B with VFD B, job coupling B, job LOS B and Cooler B, job BSOS B, Separator B, process interconnecting piping with valves between skids, job piping and job separator skid and Control unit, instrumentations and the required auxiliaries and accessories including electrical are connected to make a complete functional system for Unit B.
Tests shall be conducted step by step to ensure complete MPP package control system including field equipment. The test shall be carried out as per FAT check list and in the same methodology applied to Unit A.
But for the purpose of Lead time improvement, i.e., to ensure faster delivery, Vendor provided 2(Two) nos. proposals:
First Proposal: One complete Drive train of Unit A (Motor A, VFD A & Transformer A) for Tag No. 1-01-00-U-0901 was successfully tested (witnessed) in NIDEC’s (driver supplier) workshop. It would be acceptable to use the Motor B, VFD B & Transformer B of Unit B for conducting FAT for both the Unit A (Tag No. 1-01-00-U-0901) & Unit B (Tag No. 1-01-00-U-0902). In this way one set could be finished for early delivery and the second set can be started for FAT.
Second Proposal: Both Unit A and Unit B’s LOS and SOS were functionally tested at Sub-Vendor’s workshop’s. So one set, say of Unit A, can be included in the FAT and the other one can be finished for early delivery.
Based on the above, Vendor would exchange the Pump skid (here Pump skid means Pump & its Drive motor only) between two Units A & B. After performing FAT of Unit A (Pump skid A, UCP A, LOS A, SOS A, Separator A, VSD A, Transformer A), Vendor will just dismantled and remove Pump skid A without dismantling other components of Unit A. In that space the Pump skid B (of Unit B) will be inserted, connected with the remaining components of Unit A and FAT test shall be performed. Thus the complicated connection between Motor and VSD, VSD and Transformer, UCP and other Instrument and Electrical are not needed and handling and maintenance time is saved.
This second proposal was considered suitable.