The invention relates generally to apparatus for monitoring operation of positive-displacement pumps and, more particularly, to a liquid/gas phase detector for multi-stage positive-displacement rotary axial-screw pumps.
In conventional pumps of this type, pressure is developed from the inlet or suction port of the pump to the outlet or discharge port in near-even stage-to-stage increments. Each stage is defined as a moving-thread closure or isolated volume formed by meshing of pump rotors between the inlet and outlet ends of the pump. Pressure is developed along the moving-thread closures as liquid progresses through the pump. The number of closures is usually proportional to the desired level of outlet pressure delivered, i.e., the greater the pressure, the greater the number of closures necessary. The closures enable the pump to develop an internal pressure gradient of progressively increasing pressure increments. Properly applied, a rotary axial-screw pump can be used to pump a broad range of fluids, from high-viscosity liquids to relatively light fuels or water/oil emulsions.
However, when large volumes of entrained or dissolved gas exit in solution within the pump, the normal progression of pressure gradient development is often disrupted, adversely affecting pump performance. If large quantities of gas become entrained in the pumped liquid, the internal pumping process may become unsteady and the internal pressure gradient can be lost. The pump may also vibrate excessively, leading to noise and excessive wear.
This condition is synonymous with a more common phenomena known as "cavitation". Cavitation usually occurs when the pressure of a fluid drops below its vapor pressure, allowing gas to escape from the fluid. When the pump exerts increasing pressure on a gaseous liquid, unstable stage pressures result leading to collapse of the gas bubbles in the delivery stage.
Traditional cavitation detection has been through audible noise, reduced flow rate, and increased pump vibration. However, by the onset of these occurrences, significant changes in pump operations may have occurred and it is often too late to protect the pump from internal damage. For example, where the pump is unable to develop a normal pressure gradient from suction to discharge, the total developed pressure may occur in the last closure. This upsets normal hydrodynamic support of the idler rotors, eventually leading to metal-to-metal contact with consequential damage to the pump.
Knowledgeable application and conservative ratings are traditional protection against these conditions. However, when pumping liquids with unpredictable characteristics or uncontrolled gas content, as is often the case, frequent monitoring of pump operations with attendant labor and other costs is required to maintain normal operations. Traditional means of detecting cavitation and other operating instabilities have been found particularly unsuitable where the pump is expected to give long reliable service at a remote unattended installation, and under extreme environmental conditions.