When all the scalar quantities are considered, pressure is widely used in many industries. Basically defined as the amount of force that is applied perpendicularly to the surface of an object per unit area, pressure plays a key role in various systems. In the air systems, we have static and velocity pressure while in the pumped systems, we have dynamic, static and velocity pressure. In this article, we mainly focus on the piping systems and pressure in the pump.
Here are some key definitions to note:
- STATIC PRESSURE: in regards to a water-based system, it is a pressure defined when at system is at rest but elevated.
- VELOCITY PRESSURE: this is the pressure responsible for the movement of water. From the law of conservation of energy, water passing through a pump will always leave the pump with the same energy.
- DYNAMIC PRESSURE: it is a combination of both the static pressure and the velocity pressure
- Net Positive Suction Head Required (NPSHR): this is the pressure required for efficient operation at the suction pump
- WATER COLUMN: it exerts .433PSI per 1’-0 Μ of elevation.
Every pumped system requires pressure to operate, Normally, the difference in pressure (ΔP) is the main factor which determines the functionality of a pumped system. Therefore, it is very essential to have a technique of checking the pressure of all pumps regardless of their sizes. If you are checking pressure using pressure gauge, P/T plug or any other instrument for measuring pressure, the pressure of the fluid during suctioning and discharge when a pump is off is always the same.
The pressure(s) around the pump usually change when the pump starts working. However, the change suction pressure which usually shows the water elevation in the tower is usually minimal. The change is minimal because there is always an installation of bladder tank which is crucial in creating a zero pressure change in the system.
The amount of discharge pressure is mainly based on the resistance and other components of the pump. Logically, the pump’s role is to try to move water with the highest volume flux as much as possible for a given rotational speed and impeller size. As the pump move a given volume flux, the movement meets resistance at the system curve (pump operating point). In real life, discharge pressure and resistance can be seen in a typical mall with one exit and many people want to exit at the same time. As the number of people who want to exit the mall increases, the resistance also increases.
The movement of water through the piping is usually facilitated by the difference between the suction and discharge pump commonly referred to as head. It is also important to note that amount of pressure used is arrived at after considering other losses that can be caused by components such as orifices and valves. Therefore, suppose a discharge valve is close, it will result in a dead head. The dead head is also an important feature because it helps in determining the differential and impeller diameter which we can use to determine the flow of the fluid. For precision purposes, you will always need use three values.
As far as the pressure readings on the pump are important in determining the functionality of the pump and troubleshooting problems, it is always prudent to note the following:
- Use accurate pressure gauges that are best for the application
- Always get your suction and discharge readings from a single pressure gauge to avoid compensating for elevation
- Avoid using a snubber on your pressure gauge. Instead, you can use valves because snubbers may throw the readings off
- Foresee your readings so that you can analyze them easily
- Avoid dead heads that lasts for more than half a minute
- Ask questions when you need help
Bryan Lee, President of Zentel Tech, LLC
Originally a pipefitter and balancer, Bryan Lee’s career spans for more than 30 years. Apart from teaching for International Union and Local 597, he is usually involved in troubleshooting mechanical systems at Zentel Tech, LLC. As a veteran of all piping and pumping systems, he regularly lectures on pump troubleshooting, air conditioning, steam and other facilities.
Bryan Lee is available at 708-925-4552 or blee@zenteltech.com