What is a pitot tube pump?
The pitot tube pump is a variation on a centrifugal pump. It is unusual in that the casing rotates and not the impeller. In fact, the pump does not utilise an impeller. It uses a fixed pitot tube located within the rotating casing.
They are single stage pumps designed for low flow, high head applications. The pitot tube design produces a stable, pulsation free flow. The pitot tube pump is totally, hydraulically stable and can operate with a minimal continuous bypass flow at shutoff indefinitely and at any flow point throughout the total head curve range with no wearing or damaging effect to the pump. The reason for this unique benefit is that all radial forces tend to be balanced within the rotor, and axial thrust is solely a function of suction pressure. Radial and axial forces applied are independent of flow rate. Thus, the pump can operate at design point to shut-off free of shaft deflection or added thrust load applied to the bearings.
It has a similar performance curve to centrifugal pumps but can operate reliably at any point on the performance curve unlike centrifugal pumps which naturally have a more restricted operating range down to minimum safe flow.
HOW DOES IT WORK?
Liquid enters the pump via the suction line which acts as the distribution manifold. The liquid then passes over the mechanical seal, which is only under suction pressure, and enters the rotor cover where it is accelerated to a speed identical to the rotor speed
The liquid ring travels at the same peripheral speed as the rotor and this moving fluid annulus develops a velocity head. A stationary, airfoil-shaped pitot tube is placed inside the rotor assembly, and has a circular opening located close to the inside of the rotor. The pitot tube works in two ways: firstly, the liquid enters the pitot tube at high velocity under the centrifugal pressure created by the rotor and secondly, much of the high velocity energy is converted into pressure as the liquid passes along the diffuser section of the pitot tube.
Using this operating principle, high discharge pressures can be obtained in a single stage process. The pump generates a pulsation free flow and has a stable head curve over a wide flow range.
- Head changed by speed change
- Wide operating range up and down the curve
- Large flow changes by switching pitot tubes
- Low minimum flow (smooth, pulsation free)
- Simple control equipment, no need for relief valve, by-pass pressure unloaders, pulsation dampeners, minimum control valves
- Simple design; only two basic working parts, rotor and pitot tube
- Bearings isolated from pumped fluid
- Resistance to system transient e.g. fast operating valves, suction loss, standby changeover
- Inherent in design e.g. number of parts, low speed
- Conventional horizontal arrangement and standard electric motor with in-line manifold available
- Seal operates under suction pressure, low differential pressure generally prolongs life
- Unlike conventional centrifugal pumps, the pitot tube pump will not seize if run dry by a loss of suction or if operated with a minimal continuous bypass flow against a closed discharge valve. The mechanical seal is not mounted to the pump drive shaft, therefore, seal failure temperature rise is not transferred to the critical drive shaft/bearing area. The design does not incorporate wear rings or any close shaft tolerances which would be subject to heat expansion and drive shaft seizure.
A single stage pump with only two basic working parts;
- a rotating case and
- a stationary pitot tube.
The pump has only one wearing, rubbing part and that is a mechanical seal that sees only suction pressure. Seal leakage due to seal failure vents to atmosphere. The mechanical seal is isolated from the bearing pedestal, minimizing the risk of seal leakage contaminating the bearings. With minor seal leakage, the pump can be kept in service with a damaged seal to meet the critical demands of daily production.