Water Pressure vs Flow Rate: Difference/Relationship

What is the Relationship Between Flow Rate and Pressure?

There are people who use water pressure and flow rate interchangeably but the two are very different. So, what is the difference between water pressure and flow rate.

Water pressure describes how hard water is flowing out of a pipe in pounds per square inch (psi) while flow rate describes how much water is flowing out of the pipe, in gallons per minute (GPM).

water-pressure

Water pressure is therefore the pounds of force being exerted by the water on the pipe, while flow rate is the gallons of water passing through the pipe every minute.

Pressure is generated by gravity or a pump. That is why most cities have huge tanks on top of a hill to supply water to the entire city. As the water moves flows down from the tank, it builds up enough pressure to flow to each house.

In case you want to move water upwards (against gravity), you will need a pump. That is why people who use water from a well will need to install a water pump.

Water pressure affects flow rate directly but flow rate does not affect pressure. An increase in water pressure results in an increase in flow rate and vice versa. If you want to regulate the flow rate of your faucets or shower heads, you adjust the water pressure accordingly.

Water Pressure vs Flow Rate

Pressure is defined as the continuous physical force exerted on or against an object by something in contact with it. In other words, you can say that pressure is the force acting on a given area.

That is the reason the units for pressure is pounds (force) per square inch (area). If for instance the pressure of water flowing out of a faucet is 60 psi, it means there are 60 pounds being applied on every square inch of the pipe.

Flow rate on the other hand is defined as the volume of a fluid which passes per unit time. In the United States, flow rate is expressed in gallons per minute but in some countries like United Kingdom flow rate is expressed in liters per minute or even cubic meters per hour.

Water flow rate is therefore the amount of water flowing out of a pipe in a given time period. If for instance your shower head’s flow rate is 2.5 GPM, it simply means that there are 2.5 gallons of water flowing out of the shower head per minute.

How Does Pressure Affect Flow Rate?

The pressure of water flowing in a pipe is directly proportional to the flow rate of water. Increasing the water pressure will result in an increased flow rate while decreasing the water pressure will result in decreased flow rate.

So, how do you know that you have low or high pressure in your house? If the water pressure is low, water will simply trickle out of the faucet and it will take longer to fill a glass since the flow rate will be low as well.

On the other hand, if the water pressure is high, water will gush out of the faucet and fill a glass in a split second. In this case, the high pressure translates into a higher flow rate.

Does it therefore means that an increase in water flow rate results in an increase in water pressure? That is where it can get confusing.

I like to see pressure as the cause and flow rate as the effect. In a mathematical equation you would say that flow rate is function of pressure but pressure is not a function of flow rate.

Pipe Diameter vs Pressure vs Flow Rate

pipe-diameter

Does water pressure increase or decrease if the diameter of the pipe is increased? And how does increasing a pipe’s diameter affect the water flow rate?

To be precise, imagine water with a pressure of 60 psi is flowing in a pipe with a diameter of 2 inches. What happens to the pressure and flow rate of the water if the diameter of the pipe is increased to 4 inches?

The pressure of water in a pipe is affected by the size (diameter) of the pipe. If the diameter of the pipe increases, the pressure decreases. On the other hand, if the diameter of the pipe is reduced, the water pressure increases.

In the above example, if the diameter of the pipe is increased from 2 to 4 inches, the pressure of the water will reduce to maybe 40 psi. Why is this though? How does the size of a pipe affect its pressure?

Remember from the definition of pressure we defined it as the amount of force working on a given area. That is why to get pressure we divide force by area and hence the units (pounds/square inch).

If we reduce the size of the pipe, it means that the water is now acting (or pushing against) a smaller area. From the equation Pressure = Force/Area, reducing area means an increase in pressure.

Using the same formula with the example above, increasing the size of the pipe from 2 to 4 inches means that the water will now be acting on a bigger area. Increase in the area being acted upon by the water results in a decrease in pressure.

So, how does increasing or reducing the size of the pipe affect the flow rate?

As I mentioned earlier, an increase in water pressure translates to an increase in flow rate. We have also established that increasing the size of a pipe results in a reduction in water pressure.

It is therefore correct to conclude that an increase in water pipe diameter results in a reduction in flow rate and vice versa.

How to Check Your Water Pressure

The normal water pressure in most homes is between 40 and 60 psi. Beyond that the pressure will be too high that it can cause premature failure of appliances while below that it will not be sufficient to sustain the entire household.

If you suspect your water pressure is too low or too high, the only way you can be sure is by checking the pressure. You can thereafter adjust it accordingly.

You will only need a pressure gauge for this test. Screw the pressure gauge on an outside faucet (or any other faucet bib). The faucet needs to be connected to the main water line (which serves the house) and not an independent line.

While carrying out this test, make sure that all the other faucets are turned off and that no appliance is running. Washing of hands or even showering will distort the figures.

Turn on the faucet where you have connected the pressure gauge fully and check what the water pressure is.

If the pressure is within the allowed limit, the problem is with your house pipes. They could be clogged or you could be having leaks.

Sometimes faucet aerators and shower heads are clogged by mineral deposits and pipe corrosions restricting the flow of water. Check out this post on how to clean them and improve the pressure.

If you use water from a well, things are a different for you compared to folks who use municipal water. Check out how to improve your water pressure in this post.

How to Adjust Water Pressure

PRV

If after checking your water pressure, you have noticed that it is lower than it should be or you just water to increase or reduce it, you can do that on your own without calling a plumber.

The first thing you will need to do is to locate your pressure reducing valve (PRV). The pressure reducing valve as its name suggests allows you to reduce or increase the water pressure in your house.

Your PRV will be located very close to the main shut off valve either on an outside wall or inside the house in the basement, crawlspace, garage or utility room. It is made from brass and looks like a dome with an adjustment screw/bolt at the top.

  • Start by connecting the pressure gauge to the faucet. It will allow you to monitor the pressure as you manipulate the valve.
  • Use a wrench to loosen the nut holding the adjustment screw in place.
  • Now use a flathead screwdriver to turn the adjustment screw. To increase the water pressure, turn the screw clockwise. Turn the screw counterclockwise to reduce the water pressure.
  • Keep checking the pressure gauge till you get your desire water pressure.
  • Don’t forget to tighten the nut after adjusting the pressure.

And basically that is it about the relationship between water pressure and flow rate. I hope this guide was helpful.

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