Pitot Pump
In the previous post, low-flow high head application was introduced as one of the situations to
choose reciprocating pumps. There was also a discussion on the low-flow
high-head situations when selecting a high speed integrally geared centrifugal
pump is preferred. Here another option for such an application is addressed.
Pitot
pumps are considered to be a competitive option by many engineers in industry. Pitot
pump is a specially designed centrifugal pump which extends the specific speeds
of centrifugal pumps down to 50 o 350 (US units), namely low-flow high-head
applications. Single stage pitot pumps with capability of delivering flows up
to 800 gpm with a head up to 5500 ft have been successfully utilized.
1. Pitot
Pump Design
An example
of a pitot pump is shown in below figure. It comprises a closed rotating casing
with a stationary pitot tube that extends into the rotating case along the axis
of rotation with its inlet positioned near the maximum inner diameter of the
casing. The liquid enters casing along the axis of rotation and picks up
momentum as it passes through the enclosed radial vanes of the impeller into
the rotating casing. The liquid is spun up to approximately the full rotational
speed of the casing. It then impacts the inlet orifice of the pitot tube near
the periphery of the rotating casing (where the pressure and rotational
velocity of the liquid mass are the greatest). The liquid is discharged through
the inner passageway of the pitot tube and out of the pump.
Pitot pump
[Source: Roto-Jet High Pressure Pitot Pumps, Weir Specialty Pumps,
www.weirsp.com]
2. Pitot
Pump Performance
The head
developed by a pitot pump is the sum of two components; the static pressure
head created by centrifugal force and the velocity head. This sum will be
approximately 1.6 times the head produced from a conventional centrifugal pump
of the same size and speed.
Total
developed head by a pitot pump can be calculated from the below equation:
where r is centerline radius of the pitot tubr inlet (in) and N is
rotational speed (rpm). This
equation is not significantly different from the theoretical head produced by a
conventional centrifugal pump. It is the combination of more effective
conversion of centrifugal and velocity heads developed by the pitot pump and
minimal friction that allows this type of pump to develop high heads at
relatively modest speeds and at good overall efficiencies.
There are two methods
of changing the performance characteristics of the pitot pump; changing the
size of the pitot tube and changing the speed at which the pump is operated. In
order to represent both of these methods, a typical performance curve is
generated for each pitot tube size with a series of head and power curves for
each of the common operating speeds (see below figure).
Pitot pump
typical performance curve [Source: Angle, Roudnev, Application of the Pitot Pump,
Tutorial on Special Purpose Pump, Proceedings of the 14th International
Pump Users Symposium, pp144-149]
Pitot pump
is suitable for operation over a wide flow range. It can be safely operated at
any point on its curve from full flow to shutoff. It can be operated at its minimum flow
indefinitely without any damage.
The head
produced by the pitot pump can be easily adjusted by changing the speed at
which the pump is operated. Pitot pumps are commonly driven either by electric
motors controlled by a VFD, through a gear box used as a speed increaser or by
V-belts and sheaves.
Reference: Angle,
Roudnev, Application of the Pitot Pump, Tutorial on Special Purpose Pump,
Proceedings of the 14th International Pump Users Symposium,
pp144-149
