Downhole Pumps with Turbine Drive

V. M. Voronchikhin

(Presented by the scientific seminar of the Department of Mining Mechanics)

The design institute TsNIIpodzemshaxtostroi has developed downhole pumps N-1m and BN-15×4 with turbine drive.

The N-1m turbopump is intended for pumping water from the face directly into a water collector when sinking vertical mine shafts with an inflow of up to 20–25 m³/hour; the second turbopump, BN-15×4, is intended for bailer dewatering with an inflow of up to 8–10 m³/hour. The turbopumps may be used for local dewatering when driving horizontal workings, when digging ditches, pits, and in other earthworks. The design of the pumps permits their normal operation both when fully submerged in water and when only the suction screen is submerged; their low weight and dimensions make it possible to carry them conveniently to any place in the working.

The Department of Mining Mechanics of the Tomsk Polytechnic Institute conducted tests of experimental models of these pumps on a special stand installed in the mine-dewatering laboratory. During testing, the turbopump was placed in a sump with a constant water level of 0.75 of the height of the pump casing. The pump capacity was determined by filling and subsequently weighing a tank with a capacity of 0.1 m³, with time measured by a stopwatch. The total head produced by the turbopump was determined by summing the readings of the manometer with the velocity head and the vertical distance from the water level in the sump to the point where the manometer was installed. The revolutions of the turbopump shaft were measured with a vibrotachometer. The consumption of compressed air by the turbine engine was measured by means of an orifice plate and a differential manometer installed in accordance with Rules No. 169. The temperature and pressure of the air before the engine were measured with a thermometer and a spring manometer. Stabilization of the temperature regime of the turbine engine was achieved by preliminary operation for at least 30 min at constant working pressure, and only after this was the turbopump tested without stopping.

During the test, 7–9 operating conditions were established, and for each of them readings were taken from calibrated instruments.

The characteristics of the turbopumps were reduced to normal test conditions:

temperature of compressed and atmospheric air . . . 20°C,
atmospheric pressure . . . . . . . . . . . . . . . 760 mm Hg,
specific weight of atmospheric air . . . . . . . . 1.2 kg/m³;

The results of testing the turbopumps at compressed-air pressures of 3, 4, and 5 ati are presented in the characteristics (Figs. 1 and 2), which do not coincide with the nameplate data (Table 1).

From the characteristics of the turbopumps and Table 1 it is evident that the nameplate capacity and head of the turbopumps can be obtained only at an operating air pressure of 5 ati; in this case the air consumption of the N-1m turbopump will be 7.08 m³/min, and that of the BN-15×4, 1.85 m³/min.

Thus, the obtained air consumption is higher than the nameplate value by 12% for the N-1m turbopump and by 85% for the BN-15×4 turbopump. The consequence of this is a low efficiency, especially for the BN-15×4 turbopump.

The second shortcoming of the turbopumps is that they can operate only under load. In the absence of water (“idle” operation), the turbine sharply increases its speed of rotation; “runaway” occurs, followed by failure of the pump bearings and turbine. In connection with this shortcoming, “idle” operation of the pumps is impermissible.

Fig. 1. Characteristic of the N-1m turbopump
1 — at a pressure of 5 ati and an air consumption of 7.08 m³/min; 2 — at a pressure of 4 ati and an air consumption of 5.87 m³/min; 3 — at a pressure of 3 ati and an air consumption of 4.65 m³/min.

Table 1

The low efficiency of turbodriven pumps may be due to imperfections in the design of the turbodrive or of the pump, or to both.

To determine ways of improving economy, it is necessary to test the turbodrive and to separate the efficiency of the unit into the efficiencies of the turbodrive and of the pump.

The second shortcoming, i.e., limiting the rotation speed of the turbodrive during “idle” operation of the pump, can be eliminated by:

1) a load fan;

2) a centrifugal-load regulator;

3) a hydraulic regulator.

From the theory of turbomachines it is known that the power of a fan increases in direct proportion to the cube of the shaft speed; therefore the load fan will absorb little power during normal operation of the turbopump, and the full amount during “idle” operation. The maximum possible rotation speed during “idle” operation is determined by the strength of the bearings and of the pump turbine; consequently, the parameters of the load fan must be coordinated with the rotation speed during normal and “idle” operation. The parameters of the load fan can be determined by analogy with the calculation of pinwheels, taking into account the specific conditions of operation of the load fan in a closed volume.

Fig. 2. Characteristic of turbopump BN-15.
1—at a pressure of 5 atm and an air flow rate of 1.85 m³/min;
2—at a pressure of 4 atm and an air flow rate of 1.56 m³/min;
3—at a pressure of 3 atm and an air flow rate of 1.21 m³/min.

The advantage of this method of limiting the rotation speed lies in the simplicity of the device and in the absence of any transmission mechanisms, since the blades of the load fan are mounted directly on the disk of the turbodrive.

However, with this method of regulation, and with a twofold increase in the number of revolutions under “idle” running, the economy of the turbopumps is inevitably reduced, since during normal pump operation the load fan will take 12–15% of the rated power of the turbodrive.

In a centrifugal-brake regulator, the rotation speed when the pump changes over to “idle” operation is limited by the friction of the shoes against the brake disk.

This type of regulation is more economical than the first method. But the centrifugal-brake regulator has rotating parts and, moreover,

when the rubbing surfaces are worn, the regulator may fail to operate.

To limit the rotational speed, we propose a hydraulic regulator. The operating principle of the hydraulic speed regulator (Fig. 3) is as follows.

During normal operation of the turbopump, water from the pressure pipeline enters the upper part of the body 1 of the regulator and, overcoming the resistance of spring 2, presses diaphragm 3 into its lower position. In this case spool 4 opens the regulator inlet port to its full cross-section.

Fig. 3. Hydraulic speed regulator.

When the turbopump switches to “idle” operation, the pressure on the diaphragm becomes almost equal to atmospheric pressure. In view of this, the spring presses the diaphragm into the upper position and the spool shuts off the regulator inlet port.

Operation of turbopumps with a hydraulic speed regulator can be automatic. For this it is necessary that the spool not shut off the inlet port completely. In this case the turbine and the pump impeller will rotate at a speed somewhat higher than normal. As soon as water appears, the turbopump will first reduce its rotational speed due to the load of the pump, and then, under the gradually increasing water pressure on the diaphragm, the spool will open the inlet port and the turbopump will change over to normal operating conditions. The regulator is adjusted by means of regulating screws 5 and 6.

From a comparison of the various methods of regulating the rotational speed of the turbine drive of pumps, it is evident that the last method is simple, economical, and does not require constant supervision of the pump operation.