pump and control philosophy upgrade: overview of the upgrade to
terminal pumping station in Oxford with accompanying lessons
by Alqayam Meghji & Peter Glass
drained down with new pump & guide rails - Courtesy of
of the Thames Water AMP5 programme of works, Optimise was
instructed to carry out an upgrade to the terminal pump station
feeding Oxford STW located in Littlemore, Oxford. The terminal
pump station handles flow from a population of 200,000 and the
condition of the station was of considerable concern to the
Environment Agency, local residents, South Oxfordshire Council,
and Oxford City Council. The requirements of the new design were
to automate the entire site and maintain pass forward flow at
the permitted level. Littlemore SPS is a terminal pumping
station and as such it was imperative that the site remain in
operation throughout the duration of the works.
The existing site had 14
(No.) pumps spread between three stations; 6 (No.) in the high
level pumping station wet well (HLPS); 6 (No.) in the deep shaft
pumping station dry well (DSPS); and 2 (No.) in the storm shaft
pumping station wet well (SSPS). The existing DSPS storm pumps
had each been fitted with a throttle section on their dedicated
rising mains which, over time, had resulted in increased levels
of wear on the pumps and unnecessary expenditure on energy
The site has multiple
hydraulic connections between the pump stations to allow
manipulation of flow where pumping capacity allows. The HLPS can
overflow to the SSPS via a storm overflow chamber and weir
allowing the DSPS pumps to take the flow. Conversely the SSPS
can pump to the HLPS via the storm pumps in the event of a loss
of pumping capacity in the DSPS.
The DSPS operates as a
single dry well from which flow is pumped through an 1100mm
diameter rising main. The HLPS has the ability to operate as two
separate or one single wet well that pumps flow via a discharge
manifold with the rising main from the DSPS. Downstream of this
manifold the rising main bifurcates into twin rising mains that
carry the flow to Oxford STW.
DSPS flowmeter chamber - Courtesy of Optimise/MWH
The upgrade to the
station required the following:
Removal of 4 (No.)
existing pumps weighing over 5T each.
Installation of 4 (No.)
new pumps: 2 (No.) in the DSPS with straight spool pipes to
replace the throttles, 2 (No.) in the HLPS.
4 (No.) new starters: 2
(No.) in both the DSPS and HLPS.
6 (No.) new Pumpsmart
variable speed drives: 2 (No.) DSPS, 4 (No.) HLPS (the DSPS
already had two variable speed enabled drives for existing
All associated cabling
and mechanical fittings such as guide rails and pipework.
3 (No.) new ultrasonic
level sensors: 1 (No.) DSPS, 2 (No.) HLPS.
7 (No.) new float
switches: 3 (No.) DSPS, 4 (No.) HLPS.
3 (No.) new hot tapped
flowmeters: 1 (No.) on the DSPS discharge rising main, 2 (No.)
on the twin rising mains).
1 (No.) new ICA panel
with associated HMI and software.
A new control philosophy
was also implemented to automate the operation depending on the
levels and flow rates from each pumping station on the site.
The new pumps in the DSPS
and HLPS were designed to handle half of the consented flow each
thereby enabling the DSPS or HLPS, on its own, to pump the
entire consented flow to Oxford STW in the case of high incoming
flows. It was recognised that this situation would be a rare
occurrence and as such the Pumpsmart units were employed to
allow for variable speed. This means that between the smaller,
existing pumps and the newer larger pumps, a wide range of flows
can be accommodated during dry weather flow periods without
exceeding the recommended number of stop / starts per hour. The
pumps were supplied by Xylem as were the Pumpsmart units. The
installation of the new pumps, junction boxes and associated
cabling was carried out by AVRS.
A new ultrasonic was
installed in the DSPS as well as three float switches. Each well
in the HLPS had a new ultrasonic and two new float switches
fitted. These were again installed and cabled by AVRS.
Courtesy of Optimise/MWH
Rising main flowmeters
Courtesy of Optimise/MWH
As the site could not be
taken offline for the works to take place, careful scheduling
and H&S precautions were taken to ensure the safety of the
workers on site.
The project began with
the construction of a new flowmeter chamber around the 1100mm
diameter rising main from the DSPS. This required digging to a
depth of 5m using sheet piling and Groundforce shoring
equipment. During the excavation three cables were uncovered and
the water table was met. These two issues meant that the
original design of the chamber had to be modified to facilitate
construction of the chamber whilst maintaining the ability to
access the pipe and install a flowmeter.
Following on from this,
and bearing in mind that the station could not be taken offline
to install inline flowmeters, a decision was taken to install
hot tapped, insertion probe flowmeters supplied by Nivus and
installed by Z-Tech.
The installation of new
pumps and associated equipment within the DSPS encountered one
issue that caused delays to the project. The centre lines of the
existing suction and discharge pipework were aligned to an older
and much larger pump. This was not picked up during the initial
scoping and surveying for the project as the pumps are located
18m below ground and require confined space training to enter
and carry out work. As such when the new pump was lowered into
place it was found that the original plinth was too low and
wide, and the suction and discharge flanges were misaligned from
those of the new pump. This required specially fabricated spool
pieces as well as brackets to raise the pump and allow the base
plate to be fitted.
In order to carry out the
pump installation at the HLPS, the station was isolated and
flows were allowed to back up and overflow to the SSPS to be
handled by the DSPS. This necessitated a partial commissioning
of the operation of the DSPS to ensure that it could handle the
increase in flows without causing a premature spill.
Schematic of Littlemore SPS - Courtesy of Optimise/MWH
Click to enlarge
Prior to installing the
pumps in the HLPS the new drives, instruments and associated
cabling were installed. This was done to reduce the time that
the HLPS would have to be taken offline to install the pumps and
guide rails. However this posed H&S issues. The HLPS was a
source of odour on site due to the turbulence caused by the
inlet flow path. This meant that workers in the vicinity had to
be extremely vigilant with respect to H2S exposure. Gas monitors
were worn at all times and escape sets were also on hand.
Initially there was an occasion when a gas alarm went off at the
lower exposure limit. Work was immediately stopped and the area
was vacated. Subsequently ventilation fans were brought in to
remove the H2S from the working area, thereby allowing the
installation to continue.
The installation of the
new Pumpsmart units in an existing kiosk required the creation
of a new cable trench. This was unforeseen at the design stage,
and as such was designed on site and constructed with the input
of the team who would be installing the cables. The input from
the installation team was invaluable to creating a trench
appropriately sized and positioned to allow for ease of
installation, maintenance and future access. After the cables,
junction boxes, drives and instruments were installed in the
HLPS, and there was confidence in the ability of the new pumps
in the DSPS to handle the increased flow, the HLPS was isolated,
drained down and the pumps installed. For the installation of
the guide rails and brackets a manrider was used with all
appropriate means of rescue in place.
Once the drives, pumps,
instrumentation and cabling were installed the HLPS was then
commissioned as a stand-alone station in a similar manner to the
DSPS. This ensured that the control philosophy was functioning
as intended before moving on to commission the site as a whole,
testing the interdependencies of the three pumping stations.
spool - Courtesy of Optimise/MWH
All electrical work
carried out around the MCC and starters was permitted by a TW
SAP (senior authorised person to permit and oversee HV/LV
electrical work) and carried out by Boultings. The electrical
permits and scheduling of required resources were crucial to the
project as the installation of new starters was on the critical
path for bringing the stations to an operational state.
The installation of the
new control panel, control philosophy, and integration with
existing control equipment was carried out by Boultings. The
testing of alarms was carried out by JRP and the commissioning
by LJR Engineering.
During dry weather flow
the site operates on levels within the wet wells, ramping the
pumps up and down as required. The levels chosen have been
slightly altered during commissioning to ensure that the inlet
works at Oxford STW receives a steady inlet flow as opposed to a
pulsating one that can impact the works.
Under conditions where
the site is pumping the consented pass forward flow and the
level in the wet well reaches a pre-determined set point, the
control begins to ration the pumping from each well. The ratio
of flow from the stations is set at 1:5, HLPS:DSPS. The reason
for this is that modelling showed that flows into each of the
pumping stations is apportioned as such over most operating
bracket - Courtesy of Optimise/MWH
The works have been
completed, the site is operational and is functioning as per the
control philosophy, and all project stakeholders are pleased
with the outcome of this project. For future learning the issues
that caused delay have been given below:
surveys and understanding of the layout of existing equipment.
Lack of resource
availability to perform crucial shut downs to parts of the site.
Integration into new
operating systems of existing equipment that did not perform as
and publishers would like to thank Alqayam Meghji,
Mechanical Engineer with Optimise/MWH, and Peter Glass,
Technical Assurance Engineer with Thames Water, for
providing the above article for publication.