a pumping station there are many factors that can affect the product such as
client needs, cost, civil constraints and design best practice with the latter
being the starting point for an initial concept. Seldom does the final
constructed pumping station reflect the initial concept but it is essential
that commonly accepted best practice is only deviated from where there is a
genuine need. One of the most commonly used design guides in the UK is the,
‘Hydraulic Design of Pump Sumps and Intakes’ by M. J. Prosser.
It is commonly accepted that “the flow
approaching the intake section should be uniform, steady and of a single phase”
(Prosser, 1977). What this means is the velocity and direction of flow should
be consistent and there should be no entrained air due to the flow conditions.
Although M. J. Prosser accepts that these ideal conditions cannot always be
achieved, the recommendations for achieving this are often taken to extremes at
the expense of feasibility and therefore requires further validation or the
development of alternative solutions.
M.J. Prosser, discusses several different
flow conditions that could produce undesirable effects on the pump and the intake.
Air Gulping – During low water level
conditions due to un-expected draw down of the pump sump, air would be allowed
to enter the pipe, gathering in large pockets inside the pipe. Figure 4.6a
shown below, gives an example where the water level of the sump has been
allowed to drop significantly below bottom water level allowing substantial
pockets of air to enter the pipe while still maintaining a full pipe flow. This
would lead to a significant drop in pump performance and would result higher
than normal levels of vibration caused by a non-uniform pressure acting on the
pump impeller. This could also result in cavitation within the pump casing due
to the sudden drop in pressure caused by large pockets of air in the pipework,
a phenomenon that will be explained later within this report.
Figure 2.4 – Image showing the air
Air Entrainment – When a fluid is from a
height above the top water level, tiny air bubbles can be introduced into the
well. If this is a particularly small well or the suction flow rate is
sufficiently high, the entrained air can be dragged into the pump suction. This
could result in loss of pump prime, increased chance of cavitation and damaging
vibrations in the pump assembly. Figure 2.5 shows a typical cause of air
entrainment where the fluid is dropped from above top water level (TWL) into
the sump. Entrained air has been known to cause a drop in efficiency of up to
15% when only 3% free air has been pulled into the suction intake (Hydrotec
Consultants Ltd, 2015).
Figure 2.5 – Image showing air
Surface vortices – Surface vortices can
cause air to be pulled into the suction of a pump resulting in reduced pump
performance can result in cavitation at the eye of the impeller. This type of
vortices is encouraged by regions of swirling and stagnant flow in the wet well
(forced vortices) and a suction intake below a minimum recommended submergence
depth (free vortices). M.J. Prosser gives a direct recommendation for the
minimum submergence depth of a pump suction intake, which is a minimum of 5
times the diameter below bottom water level (BWL) within the sump (GRUNDFOS Management A/S, 2004).
Vortices may be surface or submerged formations.
They can be classified in six different types; type one being a minor surface
dimple and type six being a fully developed air core as shown in figure 2.6 (Hydrotec
Consultants Ltd, 2015), (Hydraulic Institute, 1998). The acceptance criteria
for surface vortices can be summarized by the following statement, “Surface
vortices of type 3 or more and subsurface vortices of type 2 or more are
unacceptable” (Jones, 2014).
Figure 2.6 – Image showing surface
vortices are said to be more detrimental to pump operation as the pressure
within the vortex can sometimes drop below atmospheric pressure. This can
result in cavitation damage to the suction intake geometry and result gasses
entering the pump. Submerged vortices can be classified in three main types
ranging from a type one, which is simply swirl in the fluid flow leading up to
the intake to a type three fully developed air core. This is illustrated in
figure 2.7 (Hydrotec Consultants Ltd, 2015), (Hydraulic Institute, 1998).
Figure 2.7 – Submerged vortices
Pre-swirl – Pre-swirl
at pump intakes is not something that is pump manufacturers design their pumps
to be able to cope only to a certain degree. When designing a pump sump, the
pump manufactures assume that the flow conditions within the sump are
acceptable therefore the performance characteristics they quote depend on this.
If the velocity of the flow approaching the pump intake is Asymmetrical;
meaning the velocity profile across the sump is not even, pre-swirl can occur this is illustrated in figure
2.8 (Prosser, 1977). It is commonly stated that swirl angles should be less
than 5 Degrees or 6 Degrees if only occurring during infrequent pump operation (Jones
et al., 2014).
Figure 2.8 – Diagram showing
Swirl can form in both
the clockwise and anti-clockwise directions and will be most detrimental to
pump performance when the swirl acts in the opposite direction to the rotation
of the impeller. If the rotation of swirl does oppose the impeller rotation,
then this will act as a torque that artificially increases the head that the
pump operates against. When pumps operate at a higher head, more energy must be
transferred to the fluid by the impeller. The additional energy used will mean
the pump will have a reduced efficiency as it will be consuming more electrical
power. The effect of swirling within the pipework can increase eddy formation
and turbulence which can triple the head loss due to pipe fittings. Pre-swirl
can also reduce the Net Positive Suction Head Available (NPSHA) and
cause cavitation within pump volute casing (Jones et al., 2014). NPSHA,
is the amount of suction head, measured in metres, above the centreline of the
pump impeller. Pre-swirl and excessive velocities can reduce the NPSHA to
a value below that of the vapour pressure leading to the formation of bubbles
of gas that will ultimately explode and produce a shockwave capable of damaging
the impeller, pump casing and cause excessive vibration.
Another key piece of
literature that discusses desirable flow conditions is the American National Standard for Pump
Intake Design 1998. This standard is widely accepted and the information
contained within it is referenced quite commonly within the water and waste
water industry. The standard briefly discusses flow conditions but tends to
focus more on the geometry of suction intakes; something that M.J. Prosser
summarizes quite also. The standard briefly outlines several key ‘design
objectives’ for mump intake design. It states that “specific hydraulic
phenomena have been identified that can adversely affect the performance of
pumps” (Hydraulic Institute, 1998). The phenomena it is
referring to is listed below for further discussion.
pre-swirl of flow entering the pump
spatial distribution of velocity at the impeller eye
variations in velocity and swirl time
As the above quote
states that these phenomena must not be present to an excessive degree only it
does afford the designer some flexibility as it is unlikely that these can be
eliminated for every design.
Of the above flow
conditions, the first three will be of interest within this research. The
vortices can be predicted by the CFD software and their presence can be
visually confirmed. Pre-swirl will also be measured and the criteria for this
will be discussed later in the lit review. The standard goes on to explain that
the impact of these phenomena on pump performance varies on a case by case
basis. It also highlights the fact that such a detailed impact analysis has not
been carried out. Quantifying the amount of air entrainment within the pump
suction was an initial objective of this research, however due to complicated
nature of Two Phase CFD models this was ruled out of the scope. This will be
discussed later in the report.
Pumping Station Geometry
design guide by Prosser and the American Standard also outline dimensional requirements
for pumping stations and this will be discussed within this section of the
report. There are several main styles of pumping station commonly used,
rectangular, square and circular. For each type, the designs can vary in
complexity and size to quite a large degree and this would be impossible to fit
within the scope of this project. For this reason, only rectangular, single suction
pump sumps will be discussed. Further justification for this scope limitation
will be given within the methodology section of this report.
The design guide by
M.J Prosser is the favoured guide for information on this kind of pump sump.
The guide goes in to quite a bit of detail on the dimensions that designers
should attempt to achieve to ensure the pump intake operates most efficiently.
There are several key dimensions that Prosser outlines within the design guide.
It is explained that most pump manufacturers give these dimensions in terms of
bell mouth diameter, i.e. dimension x should be 5 x D, where D is the bell
mouth diameter and d is the pipe internal diameter. These dimensions will be
used to construct the 3D models for my research. The type of suction fitting
that should be used is also discussed. Prosser states that a Bell Mouth fitting
is commonly specified as a requirement by most pump manufactures and defines a
ratio of D/d of 1.5 to 1.8 with a dimensional tolerance of +/- 20%. The guide
also briefly discusses the impact of using a ‘sharp edged intake’ which
consists of a simple pipe with no graduated inlet. Using such a suction is
expected to cause flow separation resulting in swirl within the suction
pipework, whereas the bell mouth allows the flow to remain attached to the
surface of the pipe. Figure 2.9 below illustrates this principle (Prosser,
1977). Another reason to use a bell mouth instead of a sharp edge intake is
that the bell mouth has a head loss coefficient of around 0.04 compared to 0.5
for a sharp edge (Vano Engineering, 2012)