Limiting flow rate by pump
Updated: Jun 11, 2020
When designing the drainage for surface water you may come across the two following challenges:
1) The flow must be limited, and
2) The flow must be lifted
a) Install a pump to lift the water then add a flow controller device, or
b) Install a flow controller device then add a pump to the water, or
c) Install a pump station as the flow controller?
How does a pump system differ from a flow controller in controlling flow?
A flow controller creates a vortex in the flow at a predetermined water level, often known as top water level, which limits the flow to the desired flow rate.
A pump puts energy into a stationary body of water. Correct sizing of the pump and system gives the desired flow rate.
Flow controller characteristics
A flow controller is designed so that the flow out of the surface water drainage system does not go above a certain flow rate. A maximum level that the water will rise to is calculated by the consultants and that is used to calculate design head. See typical installation detail below.
After an initial allowance of flow reaching either to or nearly to the full flow allowed, a vortex creates a restriction. The flow is throttled backwards for a time until it then increases to the full allowable flow. See example below.
In this example, the flow increases until it reaches 14 litres per sec (l/s) at a low head of 0.4 metres (m). Once the head increases beyond this point, which can be typically during a flood event, the flow controller starts to throttle the flow. At around 11.5 l/sec and 1.0m head the flow starts to increase again. As you will notice, this is at a much steeper rate meaning much more energy is being taken out of the water at this point to reduce the flow to the desired rate. Once the swale or attenuation chamber reaches the top water level the design head is at the maximum, in this case at 1.7m and the maximum flow will be reached, in this case 14.5 l/sec.
Pump station characteristics
A pump station is designed to add the right amount of energy to the water in order to achieve the correct flow rate. To calculate this we need to understand three things:
The height the water is being pumped to
The distance it is being pumped over
Any expected curves or bends in the pipe (called rising main).
As a pump only has a certain amount of energy, the greater the height, or static head, that it has to pump the lower the flow that will be achieved and vice versa.
Before the pump starts pumping it has a hill, known as static head, to climb. As soon as it starts to climb that hill another obstacle called friction losses, comes into force. The faster the flow through a pipe the greater the friction caused. This forms what is known as the system curve, an exponential curve. The curve starts from the static head (the height of the hill to overcome) on the left and increases due to friction losses as the flow rate increases. See graph below showing both the pump curve going from top left to bottom right and the system curve going from bottom left to top right. In this example the static head is 3.50m and the total head is 6.10m at 15 l/sec.
Due to the nature of fixed speed pumps you will notice that it goes above the 15 l/sec that has been specified. There are three main ways to counter this.
1. Throttle the pipework - this has the effect of creating more friction losses to overcome making the pump curve increase in a steeper way and hitting the flow rate at an increased head.
As you see on the left the curve is slightly steeper and hits the desired flow rate of 15l/sec at a greater head of 6.3m. This option is very cost effective but is not normally recommended as it uses up energy and could cause pipe blockages.
2. Slow the pump down - this has the effect reducing the amount of energy going into the water and brings the pump down towards the bottom left of the graph.
As you see on the left several curves have been added that show the pump being slowed down by controlling the frequency of the supplied power to the pump from 50Hz down to 45Hz. This can be beneficial as it enables an exact flow to be achieved but can be costly.
3. Use an average flow rate. To achieve the desired flow rate of 15 l/sec where it is pumping at 15.3 l/sec (this level of error may be allowed for, see below), the pumps could be switched off for 15 seconds in every 20 minutes bringing the average flow over the 10 minutes to the desired flow rate. The use of a flow controller controlling the flow into the pump station by its very nature will also achieve this.
Many end users of these systems such as the water companies are aware of these challenges and may have a certain allowance above and/or below the desired flow rate which they will accept. It needs to be proved that you have put your best strategy forward in coming up with a design and we can assist with this.
Important: Where a flow rate has been given by a water company, the Environmental Agency, the Local Planning Authority, Lead Local Flood Authority, Internal Drainage Board, NHBC or other governing body these must be respected and the use of any of these methods must be submitted to them prior to discharging. We are able to assist with this should this be required.
Before choosing this pump, please be aware that the pump will not very often operate when the water level is at the top water level. Unlike flow control devices, pumps do not have the ability to create a greater flow at lower heads but the flow rate increases steadily, reducing all the time in an exponential way. When the water level is lower, the difference between the water levels (the height to be overcome or the static head) is now much greater, leading to a lower flow. A further check must now be done to ensure this flow is above the minimum velocity in the pipework of 0.75 m/sec.
As you will notice the flow rate achieved is 9.76 l/sec which through a 100mm diameter pipe flows at a velocity of 1.24 m/sec, which is above the minimum velocity, so no problems there.
How we can help
Controlling flow where it also has to be lifted can be a tricky task. We hope this small insight may help you in your daily design work and will enable you to create greater value for your clients. Our in house design team have designed 1,000's of these types of pump systems where sustainable drainage systems (SuDS) has come more into mainstream. We can design and supply these systems along with comprehensive reports showing what can be achieved. Please do not hesitate to contact us today should you have any requirements on email@example.com. We look forward to speaking soon!