Rock Street, San Francisco

Problem description

The low efficiency of
the screw pumps, due to the big clearance which is caused by the constant wear
and tear. And the high maintenance cost of the screw pumps.

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Objective

To get alternative pump(s)
that will operate in place of the screw pumps and in the same manner. The pumps
must produce the flow rate of 1440 m3/h, when in operation with the
existing submersible pump.

The solution

Criteria for pump
selection

Step 1:         Determine the flow rate

Step 2:         Determine

Step 3:
Determine the friction head

Step 4:         Calculate

Step 5:         Select
the pump

Calculations

Step 1 & 2: Total flow rate and static head

Given:

Step 3: Frictional head

;    ;

Reynold’s
number

Kinematic
viscosity of water at 40?C

From the Moody chart (Appendix A): Relative
roughness

The friction factor

Step 4: The total head

Step 5: Selecting the pump

The performance curve
below represents the chosen pump to replace the previously used screw pumps.
The solution as stated is to install 3 submersible pumps that produce about 270
m3/h flow rate. These pumps will operate together with the existing submersible
pump which discharges about 720 m3/h of water.

Figure 2: Pump perfomance curve

Pump
specifications

Manufacturer                                                  :
IHP PVT LTD.

Pump type                                                       :
Self agitating submersible slurry pump (SLD)

Frequency                                                       :
50 Hz

Flow rate                                                         :
270 m3/h at 15 m Head

No. of stages                                                               :
One

Suction size (mm)                                           :
63

Discharge size (mm)                                        :
200

Temparature                                                    :
0 – 40?C

Speed (rpm)                                                    :
1440

Motor Rating (kW)                                         :
22

Voltage (V)                                                     :
415

Phase                                                               :
3 Phase

Motor type                                                      :
Water Cooled

The lay out

The pumps will be positioned two
parralell and the other parralell to this big submersible pump. Each
submersible pump will be mounted with it’s own discharge pipe. The pipes will
be mounted on top of the screw pump wall discharging in the well at the top.
The pipe that will be used is a 200 mm diameter pipe and having a length of
about 20 m. The HDPE pipe will be used. There will be no valves in between the
pump and the discharge pipe to avoid blockages and to reduce losses in the pipe
system. The discharge pipes will be held by brackets at the top.

Figure 3: Lay out

As seen on figure 3
above, Hendrina Power Station was originally equipped with four screw pumps and
one large submersible pump. Due to the screw pumps not being efficient and also
very expensive to maintain, 3 submersible pumps are suggested to replace the
screw pumps. These pumps will add on the 720 m3/h with each having a flow
capacity of 270 m3/h. All in all the pumps will make a total flow of about 1530
m3/h. which is a bit higher than the requested flow of 1440 m3/h, so that let
us say one pump seizes, they flow will not drop that much from the required.

Figure 4 : Screw
pump house

Operation

The
original operating philosophy for the system remains. That is when the water
level is at (LL) Low-Low; there will be no pump operating. When the level is
Low (L) pump 1 will be turn on. When the level is High (H) pump 2&3 will start
to operate and with the last stage when the water level is too High that
High-High (HH), pump 4 will start. Figure 3 shows the pump references.

Process Integration

The
process will be controlled manually by the Water Treatment Plant Operators in
accordance with the Operating Procedure for the system. The original operating
philosophy will not change as a result of this modification. The alternative
pumps will only improve the availability and reliability of the existing system

Mechanical System Integration

The
pumps take suction from an open sump and discharge into an open channel. There
is no impact to any mechanical components up or downstream of the pumps.

Conclusion

The alternative pumps
will improve the availability and reliability of the existing system. In total
the pumps will give the required flow capacity.

Figure 1: waste water flow diagram

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