Advantages of twin-screw pumps for transporting high viscosity heavy oil

This article introduces the advantages of twin-screw pumps in transporting high-viscosity heavy oils. Through calculation and analysis, the impact of heavy oils of different viscosities on the efficiency of centrifugal pumps and twin-screw pumps is focused on theoretically analyzing the impact of viscosity on twin-screw pump flow and flow rates. The influence relationship between shaft power and efficiency.

The oil transfer pumps currently used are basically centrifugal pumps and positive displacement pumps, with centrifugal pumps being the majority. Due to the difference between the rated load and the actual load, the impact of viscosity on the pump efficiency and the efficiency of the pump itself, there is a large gap between the actual operating efficiency and the rated efficiency. For example, the efficiency of the dehydration pump used in Gusilian in Shengli Oilfield is 37.25%, and the efficiency of the external oil pump is 38.63%: the efficiency of the oil transfer pump (centrifugal pump) used in the 5# oil gathering station of Henan Jinglou Oilfield is only 30%, and the efficiency of the water-mixing pump is only 30.4 %, which is not only lower than the rated efficiency of the centrifugal pump but also lower than other efficient oil pumps. There is a lot of room for improving efficiency.

Compared with centrifugal pumps, screw pumps have the characteristics of high efficiency and no stirring pulsation, especially when transporting high-viscosity crude oil, and have broad application prospects in heavy oil and gas gathering and transportation systems. But the disadvantages are high prices, difficult maintenance, poor durability, etc. However, with the industrialization and serialization of screw pumps, replacing centrifugal pumps with screw pumps can effectively improve the efficiency of the oil delivery system and reduce power consumption. Especially in recent years, with the improvement of screw pump product quality, the number of applications has gradually increased. The No. 2 Joint Station of Tahe Oilfield is equipped with 7 horizontal twin-screw pumps, and the heavy oil external pipeline system is equipped with a total of 16 twin-screw pumps.

 

1. The impact of liquids with different viscosities on the efficiency of centrifugal pumps and oil transfer pumps

 

1.1 Efficiency of centrifugal pumps under different viscosities

 

Taking the WDKY horizontal split-type multi-stage centrifugal oil pump with a flow rate of 50, 80, and 100m³/h and a head of 125 and 200m as an example, the pump efficiency when transporting liquids of different viscosities is shown in Table 1.

It can be seen from the data in the table that as the viscosity of the liquid increases, the efficiency of the centrifugal pump gradually decreases. When the liquid kinematic viscosity is 150 ㎜²/s, the efficiency of the centrifugal pump drops to 60%~70% of the working point efficiency. It can be seen that centrifugal pumps are not the best choice when transporting high-viscosity oil.

 

1.2 Efficiency of twin-screw pump under different viscosities

 

Since the flow rate of the screw pump changes with the viscosity under a certain pressure difference between the inlet and outlet, the screw pump can only be selected with a flow rate similar to that of the centrifugal pump. Among the samples provided by a pump factory, three twin-screw pumps with different flow rates were selected. The pump efficiency when transporting liquids with different viscosities is shown in Table 2.

 

Table 1 Pump efficiency of centrifugal pumps under different viscosities

Viscosity/(㎜²▪s¯¹) Centrifugal pump efficiency under different flow rates (Q) and head (H)
Q=50 m³/h;H=(62.5*2)m Q=80 m³/h;H=(100*2)m Q=100 m³/h;H=(50*4)m
Clearwater 60 62 69
10 55 58.4 64.7
20 52 55.3 61.2
40 48.6 52.1 57.7
75 44.4 48.5 53.6
150 37.7 42.8 47
200 34.2 39.7 43.4
300 28.7 34.6 37.6
750 16 21.3 22.7
1500 11.3 13.1 14.1

 

Table 2 Pump efficiency of screw pump under different viscosities

Viscosity/(㎜²▪s¯¹) Pump efficiency under different screw pump
Q=53 m³/h; Δp=1.2MPa Q=80 m³/h; Δp=2.0MPa Q=102 m³/h;Δp=2.0MPa
10 63.7 57.5
20 64.4 60.3
40 64.3 62.5 68.8
75 62.7 63.8 69.5
150 60.2 63.5 68.8
300 56.9 61.9
750 51.2 57.9 62.9
1500 48.7 56.2 61.1

 

It can be seen that under the premise of transporting liquids with the same capacity and the same viscosity, the efficiency of the screw pump is higher than that of the centrifugal pump. The higher the viscosity, the more obvious the advantages of the screw pump.

 

2. Analysis of the relationship between twin screw operating characteristics and viscosity

 

2.1 Flow Rate

 

During the working process of the screw pump, a sealing cavity is formed between the two screws and the bushing. As the screw rotates, the liquid moves axially with the sealing cavity. Each time the screw rotates once, a sealing cavity moves out from the inlet to the outlet. , that is, a volume of liquid in a sealed cavity is discharged.

Therefore, the theoretical flow rate Qth of the pump is the product of the volume of liquid discharged per revolution of the pump and the rotation speed, as shown in the following formula:

Qth=KB²Ln

In the formula: B——screw diameter, m
L——screw lead, m
K——Correction coefficient, this value varies depending on the screw structure
n——Screw speed, r/min

Since the pump inevitably has liquid leakage, the actual flow rate Qreal of the pump must be smaller than the theoretical flow rate Qth of the pump. If ΔQ is used to represent the amount of liquid leakage, then Qreal=Qth-ΔQ. The factors that affect the amount of liquid leakage mainly include the pressure difference between the pump inlet and outlet, liquid viscosity, the gap between the screw and the bushing, the sealing degree of the sealing chamber, etc. For the same pump, the leakage amount is only a function of the pressure difference between the pump inlet and outlet and the liquid viscosity, that is, ΔQ=f(Δp,1/v).

So Qreal=Qth-ΔQ=f(n)-f(Δp,1/v)

In the formula: Δp——the pressure difference between the inlet and outlet of the pump, MPa
v——liquid kinematic viscosity, ㎜²/s

It can be seen that under the conditions of constant inlet and outlet pressure difference and constant rotation speed, the greater the viscosity, the greater the actual flow rate of the pump. However, when the viscosity of the liquid increases to a certain extent, its flow rate change trend gradually slows down, and finally approaches the theoretical flow rate Qth.

 

2.2 Shaft power

 

The shaft power of the twin-screw pump consists of mechanical loss power, volume loss power and effective power. Mechanical power loss includes the power consumed to overcome mechanical transmission such as screws and bearings, as well as the power consumed to overcome the friction between screws and bushings and the friction between pump shafts. The former is related to the pressure difference between the inlet and outlet, and the latter is related to the viscosity of the liquid. , the greater the viscosity, the greater the mechanical power loss. Volume loss efficiency is caused by the loss of liquid. Effective power is the work done by the pump on the liquid and is the product of the actual flow rate and the pressure difference. The sum of volume loss power and effective power should be equal to the product of theoretical flow rate and pressure difference.

Therefore, shaft power P=QthΔp+f(Δp,v)

It can be seen that under the condition of a certain pressure difference between the inlet and outlet of the pump, the greater the viscosity, the greater the shaft power.

 

2.3 Efficiency

 

According to the definition, the volumetric efficiency of the screw pump η’=Qreal/Qth

Total efficiency η=QrealΔp/3.6P

When the rotation speed and the pressure difference between the inlet and outlet are constant, as the liquid viscosity increases, the actual flow rate and shaft power of the pump will increase. Therefore, the changing trend of the pump efficiency depends on the degree of change of the actual flow rate and shaft power of the pump with the viscosity.

When the viscosity of the liquid is small, the viscosity increases, the change in mechanical power loss is small, and the actual flow rate increases, so the efficiency of the pump increases. When the viscosity of the liquid reaches a certain level, the leakage of the pump changes to a certain extent. If it is very small, it can be considered that the pump does not leak, but the mechanical friction that the pump needs to overcome is getting larger and larger, so the efficiency of the pump will gradually decrease. For the same pump, under the conditions of constant inlet and outlet pressure difference and constant rotation speed, the pump efficiency and viscosity have the following relationship curve, as shown in Figure 1.

 

pump efficiency

Figure 1 The relationship between pump efficiency and viscosity

 

It can be seen from the curve in Figure 1 that the highest point where the efficiency changes with viscosity is the optimal working point of the pump. For screw pumps of different sizes, the highest points correspond to different viscosities. Therefore, during the selection process of the screw pump, you must consider The effect of viscosity on pump efficiency allows the pump to operate near the pump’s efficient operating point.

 

Source of the article: Meng Qiuli, “Analysis of the advantages of twin-screw pumps in transporting high-viscosity heavy oil”

 

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