Application of Medium-voltage VFD on Oil Pump Sets in Oilfields

Summary
Oil pumps are the main energy consuming equipment in the operation of oil storage tanks. To meet the requirements of production technology, the oil flow needs to be regulated in practical operation with the opening of the outlet valves of the oil pumps according to the working conditions. Rather high pressure-difference is generated before and after the outlet valves of the oil pumps for this flow-regulating mode. A large quantity of energy is consumed before and after the outlet valves of the oil pumps, leading to energy waste. A set of medium-voltage VFD made by Beijing Leader & Harvest Electric Technologies Co., Ltd. is applied to the Model KDY750-75×4 oil pump set of 6KV/630KW in the South First Oil Storage Tanks. Regulation with varying the oil pump speed is implemented for different working condition, eliminating throttling losses produced for pressure difference, reducing losses of oil transmission, saving electric energy and improving technology.

I. Introduction
The South First Oil Storage Tanks of Daqing Oilfield accept the oil from the two main oil-extracting factories – the First Oil Product Plant and the Second Oil Product Plant. The total oil storage is 50×104m3 and 5 oil pump sets are equipped. The main specifications are listed in the Table 1.
Table 1 Specifications for the Oil Pump Sets of the South First Oil Storage Tanks

Since the characteristic of the oil pump is not matched with the characteristic of the oil pipelines (during the selection of pumps, it is impossible to select an oil pump matched completely with the pipeline characteristic), under different actual operation conditions, the outlet valve of the oil pump needs to be adjusted to control the flow. According to the statistics, under three different operation conditions: single pump, double pumps operating in parallel and three pumps operating in parallel, the highest opening of the outlet valve of the oil pump is not greater than 10% (if 10% of opening is exceeded, it is easier the current of the motor of oil pump will exceed the normal current, leading to the motor operates with overload). Higher pressure-difference is generated before and after the outlet valve of the pump, the throttling at the outlet valve of the pump makes a large quantity of energy losses, shortening the maintenance period and operating life of the oil pump set. The statistics of the pressure difference before and after the outlet valve of the pump under various operating conditions is shown in the Table 2.
Table 2 Statistics for the Pressure-difference before and after the Outlet Valve of the Pump under Various Operating Conditions

Note: The data in the above table is the statistics based on the actual operation condition of the South First Oil Storage Tanks in 2002, which is an average value.

As shown in the Table 2, throttling losses of 1.2Mpa, 0.7Mpa and 0.4Mpa presented averagely before and after the outlet valve under several operation mode of single pump, double pumps in parallel and three pumps in parallel for the South First Oil Storage Tanks. The throttling losses before and after the outlet valve under three operation modes (single pump, double pumps in parallel and three pumps in parallel) are:
NLi=0.278PLiQi
In which: NLi: Throttling power losses of the valve under different operation conditions, kW
PLi: Throttling pressure losses of the valve under different operation conditions, Mpa
Qi: Displacement of a single pump under different operation conditions, m3/h
NL1=0.278×1.2×750=250kW 250kW/630kW×100%=39.7%
NL2=0.278×0.7×700=136kW 136kW/630kW×100%=21.6%
NL3=0.278×0.4×640=71kW 71kW/630kW×100%=11.3%
It is known from the above calculation: The throttling losses at the outlet valve of the pump under three operation modes of single pump, double pumps in parallel and three pumps in parallel take 39.7%, 21.6% and 11.3% of the normal power.
Thus the energy waste is considerably high. Therefore, it is necessary to apply the VF speed regulation technique to oil pump sets to avoid the throttling losses at the outlet valve with fully opening valves and operating with varying frequency to meet the demands of operating condition under the same operating conditions.

II. Principle of Energy Saving with VF Speed Regulation of Oil Pump Sets
Based on the characteristic of centrifugal pumps, the regulation of the operating conditions is flow regulation mainly. There are two usually used methods for flow regulation. One is to regulate the opening of the outlet valve of the pump, and another is to regulate through varying the speed of the centrifugal pump. The former is easier to regulate, but it makes high energy-waste. Through VF of the motor of oil pump the motor speed will vary to perform the operating condition regulation. This is a feasible technical way to meet the technological conditions.
From the characteristic of centrifugal pumps, under unchanging pipeline characteristic, the variation of the performance after changing the centrifugal pump speed is determined by the following formula:
Q / Q1= n/ n1 H /H1=( n/n1 )2 N/N1=(n/n1)3
　　 in which：Q, H, N -- flow, pumping level, power when the pump speed is n；
　　 Q1, H1, N1-- flow, pumping level, power when the pump speed varied to n1

It may be known from the above equation for before and after the variation of pump speed, if the pump speed decreases a little, the input power needed by the pump will decrease considerably, thus obvious energy saving effects will be obtained. When the pump speed decreases for 20% of the normal speed, the characteristic curve of the pump is similar to the original curve. In Figure 1, when the pump speed decreases from n to n1, its characteristic curve is a curve parallel to the original curve. If the original pipeline characteristic curve is R, R intersects with-Q(n) at A, thus A is the original operating condition point. Under VF condition, when the pump speed is n1, its characteristic curve is H1-Q1(n1); since the outlet valve opens completely, the pipeline characteristic curve becomes the flatter R1(n1). R1(n1) intersect with H1-Q1(n1) at point A1, it is the new operating condition point. Q1=Q, i.e. keep the pump displacement unchanged, but the pumping level of the pump will decrease from H to H1. Therefore, under the condition of ensuring oil flow is satisfied, the saved energy for reducing the pumping level of the pump is the area of HAA1H1. This is the principle for energy saving for the oil pump.

III. Selection of Medium-voltage VFD
At present, the 6kV medium-voltage VFD is at the technical development stage and the basic principle is to change the motor speed through varying the stator’s voltage frequency, through the inverting process of AC-DC-AC. According to the technical implementation methods, medium-voltage motor speed regulation method may be divided into “High-Low-High”, “High-High”, “IGBT direct cascade” etc., in which “high-low-high” schema needs extra transformer for raising voltage and the equipment has large structure and relative low system efficiency. It has been the technique lag behind and will be superceded. The “high-high” schema uses input voltage of 6kV directly. No output transformer is needed for 6kV output and the system has relative high efficiency. At present, the VFD of this type has rather extensive application. The VFD of “IGBT direct cascade” uses IGBT component of 1700V high voltage. It has advantage of less components and taking less space. But IGBT component of direct cascade has not matured technically, so there is certain risk in technique. After investigation for the application of various VFD of 6kV in the country and abroad and consideration in economic and technical performance of various VFD, it is decided finally to apply the medium-voltage VFD of Model Harsvert-A06/076 made by Beijing Leader & Harvest Electric Technologies Co., Ltd. to the 2# oil pump set of South First Oil Storage Tanks. The VF speed regulation system has the following technical specifications:
　　※ Inverting main loop mode: cell cascade, multi level
　　※ Normal capacity: 790kVA
　　※ Normal output current: 76A, normal output voltage: 6kV
　　※ Input frequency: 50Hz±10%, range of output frequency: 0.1 Hz-50Hz, Output frequency
　　　 resolution: 0.01Hz
　　※ Power factor at input terminal (for 20% normal load) >0.95
　　※ Efficiency of VFD >96%
　　※ Overload protection: 120% normal current, 1min., 150% normal current, 3s, 200% normal
　　　 current, protect instantly
　　※ Acceleration and deceleration time: 0.1-300 adjustable
　　※ Harmonic control: input current <4%, output voltage 6%, output current 2%
　　※ Analog input and output signals of control part: 0~20mA standard signal
　　※ Communication interface of industrial control computer with outside: RS485
　　※ Protection level of case: ≥IP20
　　※ Cooling mode: Air cooling, operating environmental temperature: 0~40℃

IV. Technical scheme
Applying medium-voltage VFD to oil pump sets may produce good energy saving effect, but since the oil transmission system is an important link for the production of oil storage tanks. In addition to the high reliability requirement for the equipment, the technological feature must be taken into consideration in the technical scheme. The safety, suitability and convenience of site operation, starting, stopping and adjusting must be considered. The following technical measures are taken in the system:
　　(1) The system has manual switching function between industry frequency and VF. Once the VF
　　　 system has failures, it may be switched to industry frequency manually, leaving VF system
　　　 aside. The oil pump may operate normally during maintenance to meet the production
　　　 requirements of the oil storage tanks.
　　(2) The adjustment of the operating frequency of the system is performed with an open-loop
　　　 manual mode. Considering that the oil transmission system demands operating safely and
　　　 stably, it is unsuitable to use a closed-loop control for the system. If a closed-loop control is
　　　 used, it is easy to make the system sop automatically, or cause the whole oil transmission
　　　 system distributed, bringing adversely affects to the dispatching and control of the oil
　　　 transmission production. Adopting an open-loop manual control, through probing into
　　　 system operation for a certain time to set and adjust the VF system parameters under
　　　 different operating conditions manually, may reduce a part of initial investments and ensure
　　　 the safe and stable operation of the oil transmission system.
　　(3) The setting, starting, stopping and emergency stooping buttons at the site and the upper
　　　 level computer for real-time display of operating parameters in the control room make the
　　　 operating personnel at the site easy to operate and monitor the operating conditions of the
　　　 equipment.
　　(4) Optimizing the protective parameters of the system ensures the continuous stable operation
　　　 of the oil transmission system. It is necessary to select a protective parameters of the oil
　　　 transmission system carefully and set some protective parameters according to the practical
　　　 requirement, to avoid the oil pump stopping for the too sensitive protective system and affect
　　　 the safe and stable operation of the oil transmission system.
　　(5) Alarming functions suitable for the actual site are set in the VF speed regulation system and
　　　 the detail record function for the operating parameters, operating condition and failures are
　　　 included in system.

V. Effects of Application
On April 18, 2003, the VF speed regulation system for 2# oil pump set of South First Oil Storage Tanks was put into operation successfully. After trial operation for two months, obvious energy saving effect has been obtained. The data for comparison of operation with single pump, operation with two pumps in parallel and operation with three pumps in parallel are listed in Table 3, Table 4 and Table 5.

Table 3 Comparison of Operating Data for Single Pump (Pump Set 2#) Operation before and after Application of VFD

Table 3: data comparison of single pump (2# pump)

Time	Pump pressure MPa	Pipe pressure MPa	Pressure difference MPa	Motor current A	Frequency HZ	Speed rpm	Discharge t/h	Daily consumption kWh	Oil consumption kWh/t	Saving rate %
March, 2003	1.75	0.53	1.22	70	50	1980	650	14395	0.92	0
May, 2003	0.56	0.55	0.01	36	37	1465	646	7403	0.48	48.6

Table4: data comparison of double pumps (2# and 1# pump)

Time	Pump pressure MPa	Pipe pressure MPa	Pressure difference MPa	Motor current A	Frequency HZ	Speed rpm	Discharge t/h	Daily consumption kWh	Oil consumption kWh/t	Saving rate %
March, 2003	1.98 1.98	1.21 1.21	0.76 0.77	70 70	50 50	1980 1980	610 610	14190 14100	1.04 1.04
May, 2003	1.89 1.20	1.19 1.19	0.70 0.01	69 40	50 38.5	1980 1525	615 615	14230 8226	1.04 0.56	41.7

Table5: data comparison of three pumps (1#, 2# and 3# pump)

Time	Pump pressure MPa	Pipe pressure MPa	Pressure difference MPa	Motor current A	Frequency HZ	Speed rpm	Discharge t/h	Daily consumption kWh	Oil consumption kWh/t	Saving rate %
March, 2003	2.15 2.20 2.10	1.75 1.75 1.75	0.40 0.45 0.35	71 70 64	50 50 50	1980 1980 1980	580 580 580	14050 14100 12950	1.01 1.03 0.93
May, 2003	2.10 1.76 2.0	1.75 1.75 1.75	0.35 0.01 0.25	70 45 64	50 42 50	1980 1663 1980	590 590 590	14230 8630 12980	0.99 0.61 0.92	38.8

Note: 1. Data in Table 3, table 4 and Table 5 are an average value of the statistics based on the same condition of the month;
2. When several pumps operate in parallel, there is no single measurement for single pump displacement. Since the normal displacement is same, the oil quantity of that day ÷24hrs ÷number of operating pumps is computed as its single displacement.

From the statistics of the Table 3, Table 4, and Table 5, it is known under different operating conditions, the pump 2# operating with VF generated obvious electricity saving effect. Comparing with the same operation condition, the electricity saving ratio for the pump 2# operating in single pump is 48.6%, the electricity saving ratio for the pump 2# operating with the pump 3# in parallel is 41.7% and the electricity saving ratio for the pump 2# operating with the pump 1# and the pump 3# in parallel is 38.6%. Based on the statistics for several years past, the time for operation of single pump for the South First Oil Storage Tanks took 30% of the total operation time. The operation time for double pumps took 55% of the total operation time. The operation time for three pumps in parallel took 15% of the total operation time. Based on this, it may be obtained by calculation that comparing with the condition before VF speed regulation was applied, and the comprehensive electricity saving ratio is 43.3. If calculating at annual 350 days of operation, the electricity saving is 216.7×104kWh per year. Calculating at the electricity price of 0.465 Yuan/kWh for 6kV industry electricity, the annual electricity saving benefit is 1,000,000 Yuan and the equipment investment may be returned in a year basically.
In addition, since soft starting and soft stopping of the oil pump sets are performed with the VFD, shock during the starting is reduced, extending the maintenance period for the oil pump. Comparing with the same operation condition before the VFD was applied, the pumps operate with lower speed, and the wearing of the pump axles and bearings is reduced. Take the bearing as an example, the bearing temperature for the pump 2# for normal operation before the VFD applied is 85℃~ 90℃, but the bearing temperature after the VFD applied is only 60℃~65℃. This will extend the life of the pump bearings and mechanical seals considerably and reduce the operating noise. Therefore, in addition to the obvious direct economic benefits, better indirect economic benefits are also obtained.

VI. Conclusions
The medium voltage VFD speed regulation technology for oil pump sets is an effective technical way for energy saving of oil transmission systems. It changes the operating condition regulation with valve throttling into the operating condition regulation with varying speed of oil pumps. It features an easy way to regulate. The full opening of the pump outlet valve avoids the throttling losses at the pump outlet valve and generates considerable energy saving effect. The mechanical shock, wearing and noise of oil pumps are reduced. The maintenance period and operating life of the oil pump sets are extended. Indirect economic benefits are also obtained. Since the medium voltage VFD speed regulation technology now is at technical development stage and the initial investment is high, so for oil transmission system, medium voltage VFD with high reliability, good performance/price ratio and extensive application must be selected. In application, suitable speed regulation mode must be selected in consideration of the technology condition at the site to ensure the safe and stable operation of the oil transmission system on the basis of obvious energy saving effects.

References

1. Ru Huilin, Ed. “Introduction to Energy Saving Technology of Oil Transmission Pipeline”, Petroleum Industry Press, Beijing, 2000

2. “Technical Manual for Medium Voltage Inverter HARSVERT-A”

Brief Introduction of the Author:

Ma Mingli
Male, born in 1967, senior engineer, graduated from Southwest Petroleum Institute in 1960. MD of Southeast University, Engaging in management of oil storage and transmission in the Transportation and Sales Branch of Daqing Oilfield Co., Ltd.