Thrust Balancing & Minimum Thermal Flow in Complex Well Applications

Oil and gas production wells initially have sufficient pressure for gas and condensate to flow freely from the wellheads through export pipelines to the nearby existing gas plant under reservoir pressure. As the reservoir pressure declines, the peak gas will no longer be attainable. As a result, pumping equipment is needed to assist the recovery as the reservoir production rate and pressure decline. In one case study, the export condensate pump must cover operating conditions related to production rates and pressure deterioration without pump re-bundling or piping modification. The pumps need to handle condensate at various operating conditions. Because of gas well decay, condensate is pumped as a fluid near its vapor pressure and is subject to flash easily with slight pressure or temperature changes. The pressure deterioration in the well leads to an increase in the pump head from 940 to 1,800 meters. Eventually the gas condensate mixture becomes lean (less condensate), and the condensate flow rate falls from 71 cubic meters per hour (m³/hr) to 27.6 m³/hr.

Pump Selection

The user selected barrel-type condensate pumps that were compliant with American Petroleum Institute (API) 610 BB5. The double casing radially split multistage horizontal pumps have 16 stages in a back-to-back (B2B) configuration. Centrifugal pumps convert only a portion of the power input by the driver into hydraulic energy. The remaining energy is mainly converted into heat. These internal losses in energy depend on the flow rate and eventually get dissipated through the pumped liquid. In most pump curves, the lower the flow rate, the lower the pumping efficiency and consequently the higher internal losses. Having low flow concerns will lead to constraints against low flow rates in high pressure, high power pumps. Temperatures will rise significantly because of the ratio between the power input and the mass flow of the pumped fluid. The temperatures should then be investigated to ensure that no internal flashing occurs because of this temperature rise. Users should also check internal areas such as bushings and wear rings for a rise in temperature.

Pump Minimum Continuous Flow

The pump industry uses two terms to define the minimum possible flow rate for safe pumping operation. Minimum continuous stable flow (MCSF) is mainly related to pumped fluid recirculation that may result in cavitation and vibration. Minimum continuous thermal flow (MCTF) is concerned with temperature rise at lower flow rates. The minimum operating flow is the higher of the MCTF and MCSF. Figure 1 shows the relationship between the flow rate, pump efficiency and power absorbed. As the curve approaches lower flow rates, the efficiency drops and the internal losses increase dramatically. This behavior adds heat to the pumped fluid at these low flow rates. This heat addition should be calculated to obtain the required MCTF. Generally, MCSF prevails in most applications with much higher rates than MCTF and becomes the defining variable. In this case, however, MCTF dominated because of the following:

• Low specific heat (Cp) value. The condensate Cp is almost half of the Cp of water, so the same energy that would raise water's temperature by one degree Celsius will heat up condensate by two degrees Celsius.
• High-pressure, low-flow operation.
• The pump suction pressure was slightly above vapor pressure, which means that any loss in pressure or increase in temperature caused the pumped fluid to turn to vapor.

MCTF is a concern for the selected high head multi-stage BB5 export pumps. In this case, liquid condensate heats up as it passes through the pump stages and internal parts. The temperature rise is likely to occur in the following cases:

• through the pump's 16 stages where pressure develops.
• because of the throttling of the condensate in the clearance of the thrust balancing device.

The balancing line then acts as a passage between the axial thrust balance device and the suction side of the pump. If heated to a certain limit, the liquid condensate may flash and lead to catastrophic pump damage. This concern leads to operating constraints in light hydrocarbon applications, where the pump intake suction is directly fed from an elevated condensate separator.

Thrust Balancing Systems

Thrust balancing is a necessary part in every centrifugal pump arrangement. Thrust forces are magnified in high pressure, multi-stage pumps to a level that should be eliminated or at least reduced. Net thrust values could be enhanced by:

• the B2B arrangement provided to the impellers of the pumps.
a thrust-balancing device with a leak-off line to eliminate the residual thrust forces in the pump. This will increase the rotor dynamic stability and improve axial thrust compensation.

The balancing thrust bushing is found after the eighth B2B stage. A line is routed from the bushing's other side back to the suction. The size of the bushing must allow as much net thrust as residual thrust forces. Note that the area multiplied by the pressure difference between the suction and eighth stage discharge equals residual force. The balancing line can be routed to two conventional locations for balancing with lower pressure side: the suction drum or the pump suction nozzle. The balancing line is at the suction pressure, where it receives the liquid condensate that increased in temperature through pumping for eight stages. This combination of temperature gain and exposure to low pressure can cause the high vapor pressure condensate to flash. Mechanical seals may also cause problems, including:

• improper thrust balancing and less rotor dynamic stability.
• mechanical seal malfunction that may lead to seal failure.

The condensate temperature rise in the pump was calculated for all flow conditions. The MCTF was defined to ensure compensation for the temperature rise of the balance leakage so that the return leakage flow does not flash when it returns to the reduced pressure suction area. Table 1 shows that the MCTF calculations considered both balancing line return locations (suction drum/suction nozzle) with and without an orifice installed. The orifice installation in the balancing leak-off line enhanced the MCTF value significantly. This can be beneficial in other cases where such values are sufficient for proper pump operation. In cases where the previously listed return locations might be used, pump original equipment manufacturers (OEMs) should specify the balance flow rates. Operation teams should ensure that the line is open throughout pump operation to avoid damage. The OEM should advise the user on the maximum back pressure created in the balance line. The balance line is typically designed for a maximum pressure loss of 1.5 bars.

Balance Routing Options

One condensate pump OEM offers BB5 pumps with a design option to route the discharge of the first stage. Image 1 shows the pump arrangement installation. This means the balance leakage at the bushing will move from eighth-stage pressure to first-stage pressure (which varies in the presented cases from 3.6 to 7.4 bars) instead of suction. At the same thermal energy level introduced to the liquid condensate, the possibility of condensate flashing to vapor at high pressure (in the first stage) decreases. So the MCTF (shown in Table 2) is lower with a wider pump operation range. These examples show that the more pressure exists at the balancing line exit, the less flow is required to overcome the heating (the less MCTF). Any flow recycle is an energy waste and will decrease the process efficiency.

Balancing with the First Stage

The mechanical seal in the balancing line side is exposed to the suction pressure plus the pressure of the first stage. This raises the design pressure of the mechanical seal slightly. Because this mechanical seal is a double seal with barrier fluid and accumulator (Fluid Sealing plan 53B), operators must consider that the seals installed on the pump, drive and non-drive sides have about 10 percent difference in seal pressure. So the setting of the accumulator pressure and the recharging time are different in the two seals. The leakage rate for a mechanical seal exposed to suction plus first-stage pressure is higher than the leakage rate for a mechanical seal exposed to suction pressure only, which is around 5 percent in this case.

Conclusions

Wellheads are a challenging application for pumps because of the well pressure declines and flow rate changes throughout the well lifetime. The pump must match all the anticipated cases with no operation obstacles. The service liquid is a condensate that often flashes and is pumped near its vapor pressure. For the case mentioned previously, a multi-stage pump was selected with a B2B arrangement. Besides the mechanical and hydraulic considerations for this type of high-pressure pump, special attention should be paid to the thrust balancing. Because of the flashing nature of condensate, the thrust balancing line may also affect the MCTF of the pump. The pump OEM and contractor performed extensive analysis to understand factors affecting the MCTF. This analysis revealed that the balancing line conventional return locations may not achieve the required values of MCTF, so further enhancement is needed. One example is the installation of orifices, which showed significant improvement in the MCTF values. In severe cases where orifices cannot provide an adequate solution to the MCTF issue, balancing line is then routed to the higher pressure side (first-stage impeller), which leads to a wide separation between the vapor pressure at the balancing line inlet temperature and the line pressure. This causes a significantly lower MCTF to match the operation need.