THE INFLUENCE OF THE METHOD OF CONTROL ON THE OPERATING ECONOMY OF A DOWNHOLE TURBOPUMP

V. M. Voronchikhin

(Submitted by the Department of Mining Machines, Mine Transport, and Mining Mechanics)

In the sinking of vertical mine shafts and near-shaft workings, the downhole turbopump N-1m with a pneumatic drive, designed for a capacity of 25 m³/hour and a head of 40 m of water column, which it develops at a compressed-air pressure of 5 ati, has found wide application for pumping out water. The turbopump was developed as the first-stage pump of a two-stage drainage system for delivering water from the shaft bottom to a height of 20–40 m into the tank of a suspended pump, and from this tank the second-stage pump discharges the water to the surface or to an intermediate level.

Under production conditions the turbopump is often operated at a compressed-air pressure above 5 ati, is connected to an air line with a small geodetic height (horizontal or inclined pipelines of short length), and is also used as a pump for sump drainage. The capacity of the turbopump depends on the geodetic height of water delivery, the resistance of the discharge pipeline, and the pressure of the compressed air. In the technical literature it is stated that capacity control may be carried out by throttling the compressed air with a valve or plug cock on the supply air line [1], or by creating additional resistance with a valve on the discharge pipeline [2]. The supply of compressed air to the pump turbine is carried out through two nozzles; therefore one more method of control is possible—by changing the number of nozzles in operation.

By throttling the compressed air, it is possible to obtain a smooth change in the power of the turbine drive, its rotational speed, and the pump capacity. However, in this case part of the energy of the compressed air is lost in overcoming the resistance of the throttle.

By increasing the resistance of the discharge pipeline, it is also possible to obtain a smooth change in capacity, but with this method of control the compressed-air consumption remains constant, since it does not depend on the operating mode of the pump.

With nozzle control, the power of the turbine drive, its rotational speed, and the pump capacity change stepwise; and, with two nozzles of the turbine drive and an unchanged characteristic of the discharge pipeline, only two operating modes of the turbopump can be obtained. When one nozzle is shut off, the compressed-air consumption

Axis labels in the figure: pump capacity, m³/h; head, m of water column; shaft speed, thousand rpm; efficiency, %.

Fig. 1. Characteristics of the N-1m turbopump with one nozzle of the turbine motor switched on: 1 — at a pressure of 4 ati and an air flow rate of 2.95 m³/min; 2 — at a pressure of 5 ati and an air flow rate of 3.53 m³/min; 3 — at a pressure of 6 ati and an air flow rate of 4.1 m³/min.

decreases by a factor of two, while the power of the turbine motor changes by more than a factor of two, since the mechanical and ventilation losses in the turbine motor, which then fall on a smaller power, will lower its efficiency.

To evaluate the economy of operation of the turbopump with different methods of regulation, tests were carried out at the department with one nozzle of the turbine motor switched on at compressed-air pressures of 4, 5, and 6 ati. The second nozzle was shut off by a lead plug on the side of the inlet opening.

During the tests, the capacity, the pressure developed by the pump, its shaft speed, and the air consumption of the turbine motor were measured. The specific work performed by the compressed air was determined from the formula

\[ L_{\mathrm{ad}} = \frac{k}{k-1} R T_{\mathrm{in}} \left[ 1 - \left( \frac{p_{\mathrm{at}}}{p_c} \right)^{\frac{k-1}{k}} \right], \quad \mathrm{kgm/kg}, \]

![Figure 2 graph with axes: pump head, m of water column; specific energy consumption, kWh/m³; pump capacity, m³/h]

Fig. 2. Characteristics of the N-1m turbopump and the specific consumption of compressed-air energy when regulating the operating mode: 1 — at an air pressure in the nozzle chamber \(p_{\mathrm{in}} = 6\) ata; 2 — the same at \(p_{\mathrm{in}} = 3.5\) ata; 3 — at \(p_{\mathrm{in}} = 6\) ata and one working nozzle. I — characteristic of the pipeline when pumping water into a tank; II — the same for an inclined pipeline; III — to the tank of the suspended pump

where \(k\) is the adiabatic exponent;
\(R = 29.27\) is the gas constant of air, kgm/kg\(^\circ\)C;
\(T_{\mathrm{in}}\) is the absolute temperature of the air before the nozzle, \(^\circ\)K;
\(P_{\mathrm{at}}\) is atmospheric pressure, ata;
\(P_c\) is the air pressure in the nozzle chamber, ata.

The adiabatic (available) power of the turbomotor is

\[ N_{\mathrm{ad}}=\frac{L_{\mathrm{ad}}\cdot G}{60\cdot 102},\ \text{kW}, \]

where \(G\) is the air flow rate through the turbomotor, kg/min.

The characteristics of the turbopump, plotted after processing the test results, are given in Fig. 1. As can be seen from Fig. 1, the capacity of the turbopump at a small geodetic height and low resistance of the pressure pipeline can reach \(20\text{--}30\ \text{m}^3/\text{h}\).

The maximum efficiency values at air pressures of 4, 5, and 6 ati are, respectively, 12, 14.5, and 16%, i.e., somewhat lower than when the turbine engine operates at these same pressures with two nozzles engaged [3].

However, the operation of the turbopump cannot be evaluated solely by efficiency. A more appropriate indicator is the specific energy consumption of compressed air per unit of turbopump capacity,

\[ q=\frac{N_{\mathrm{ad}}}{Q},\ \text{kWh}/\text{m}^3, \]

where \(Q\) is the turbopump capacity, \(\text{m}^3/\text{h}\). In throttle control, moreover, the adiabatic power of the compressed air must be determined not from the air pressure in the nozzle chamber of the turbine engine, but from the air pressure in the pneumatic network.

Table 1

We shall compare the methods of control and the specific energy consumption using as an example various operating modes of the turbopump (Fig. 2). Let the compressed-air pressure in the pneumatic network be taken as 6 ati. When water is pumped into a skip bucket, the pipeline characteristic (curve I) intersects the characteristic of the unregulated turbopump at point \(A\). In this case the pump capacity is \(45\ \text{m}^3/\text{h}\), and the specific energy consumption (point \(a\)) is \(0.46\ \text{kWh}/\text{m}^3\). By throttling the compressed air to 3.5 ati (point \(B\)) and by bringing one nozzle into operation (point \(C\)), the turbopump capacity is reduced, respectively, to 34 and \(33.5\ \text{m}^3/\text{h}\). The same capacity can be obtained by increasing the resistance of the discharge pipeline (point \(A'\)).

The operating mode at point \(A'\) is also obtained when water is pumped through an inclined pipeline 60–75 mm in diameter and 50–100 m long, with a small geodetic height (curve II). With throttle and nozzle control, operating modes can be obtained respectively at points \(B'\) and \(C'\). Similar operating modes (points \(A''\), \(B''\), and \(C''\) on curve III) are obtained when water is pumped from the shaft bottom to the tank of a suspended pump located at a height of 30 m above the bottom.

The technical and economic indicators of all the indicated operating modes are summarized in Table 1.

It is evident from Table 1 that the lowest energy consumption is obtained in all the cases considered with nozzle control. Increase-

an increase in the resistance of the discharge pipeline leads to a significant overconsumption of compressed-air energy; therefore, this method of regulation should not be used, especially since the regulating valve itself increases the resistance of the discharge pipeline and, consequently, reduces the operating efficiency of the turbopump. Throttle regulation when pumping water into a bucket and through an inclined pipeline is accompanied by a decrease in the specific energy consumption, whereas when water is delivered to the tank of the suspended pump the specific energy consumption increases.

When the geodetic height is increased to more than 30 m, nozzle regulation also becomes uneconomical. In this case, however, nozzle regulation is generally inexpedient, since the pump develops a low capacity.

Thus, nozzle regulation of the operating mode of a turbopump is an economical type of regulation, especially when the turbopump operates on a short pipeline with a small geodetic height. A reduction in the air consumption by the pump’s turbine motor when the compressor station has low capacity will favorably affect the operation of the other downhole mechanisms. In addition, operation of the turbine motor with one nozzle switched on reduces the danger of the turbopump “running away” at “idle” operation in the event of a faulty safety valve.

Unfortunately, the wide use of nozzle regulation is limited by the fact that the turbopump has only two nozzles of the same diameter; therefore, shutting off one nozzle is accompanied by a sharp decrease in capacity. If nozzles of different diameters are made, for example 7.5 and 9.0 mm (instead of 8.5 mm), then, at one air pressure, depending on which nozzles are switched on, three different turbopump characteristics can be obtained. If partial throttling of the compressed air is also applied, it becomes possible to regulate the capacity of the turbopump smoothly and economically over wide limits.

REFERENCES

1. M. Sh. Karpilevich. New drifting turbomachines with a turbine pneumatic motor. Ugletekhizdat, 1957.

2. Downhole turbopump N-1m. Pump data sheet and brief instructions for maintenance and operation. TsBTI, 1963.

3. V. M. Voronchikhin. Downhole pumps with a turbine drive. Izv. TPI, vol. 113, 1960.