My water company is installing a new pump in the area and has warned us that this will increase our pressure by 45 psi, not to 45 but 45 in addition to what we have. I've checked the pressure at a rear outside faucet and it registered 70 psi. I checked it by the wash machine and it registered 45 psi. I have a pressure relief valve at the point of entry. Will the increase to 90 or more psi cause any problems and why the big difference in pressure readings?

I'm assuming you mean you have a pressure reducing valve at point of entry(not a pressure relief valve). Only reason I can think of for the large difference in pressure between outside faucet and washer is that the outside faucet may have been supplied before the pressure reducing valve. When the new pump is installed, I would double check pressure at both areas. You may need to adjust the pressure reducing valve to lower pressure down(max of 80psi in my area, but I would take it a little lower than that, say 50 to sixty psi). IF your outside fuacet is still reading way high(and 115psi would be way to high), you will need to either repipe faucet supply so it connects downstream of reducing valve, or a add a second reducing valve for any lines that aren't regulated by the main prv.

MGD has a terrific answer. I would only add this. Make sure you are not using water anywhere when you check for pressure. If you check the washing machine while the kitchen faucet is on, it will affect your reading.

Something sounds a little strange about all this. Simply adding a new pump would not, I think, increase a water system's pressure. I thought that was generally not determined by the pump, but by a water tower or pressurized system of some sort. Not doubting your story, but just saying it seems unusual.

OK, let's assume a pressure regulating valve, so it shouldn't change anything. Try to regulate the pressure to about 55 PSI max. It saves wear an tear on your valves, etc.

Now an explanation of the higher pressure. Kind of simple with this intuative example.

Take a piece of 3/4 pipe and attach a 1/2 pipe to it pointing down. Now take a dowell the same dimension of the 3/4 pipe and put a 10 lb weight on it.

You now have a system delivering 10 PSI neglecting gravity and the weight of water.

So, in this case you have 10 lbs of force delivered across the cross sectional area of the 3/4 pipe.

Well guess what? That same 10 lbs of force is being delivered to the 1/2" pipe too. The cross sectional area is smaller, so 10 lbs/(a smaller number than the cross secional area of 3/4" tube) is a bigger number.

The distribution at the washer is not likely 3/4 and therefore the pressure is higher because the diameter is smaller.

OK?

Every one has stated very well and I will add mine also
What is considered normal water pressure? There really is no such thing as "normal" water pressure. Since water pressure is influenced by the elevation of the reservoir that serves your neighborhood versus the elevation of your water meter, water pressure varies from property to property. Water pressure in the District typically ranges from 45 - 125 psi; however, typical residential systems are designed to function best under a pressure of 40 - 60 psi. If the water pressure entering your home exceeds this level, you should install a pressure regulator in the line to reduce the pressure to an acceptable range. This protects your equipment and piping in your home from damage from excess force exerted by the water. Symptoms of high pressure includes "clanging" or "rattling" of pipes that occur when a faucet, dishwasher, or washing machine shuts off; water heater pressure relief valve malfunctions, and reoccurring leaks. The symptom of low pressure is typically a noticeable reduction of flow.
If you have a pressure regulator valve installed you can adjust the pressure by turning the adjusting screw Counterclockwise see the images, Thanks.

John

KISS, I think you are 100% incorrect. The system pressure is measured in pounds per square inch, not in pounds per size of pipe. So, the pressure measured in a 1/2" pipe will be the same as measured in a 3/4" pipe, since it is measured as pressure per square inch. Now, the TOTAL pressure in the 1/2" pipe will indeed be less, since it has less area. But the measure is still in pounds per square inch, not in pounds per amount of surface area. If you run the 1/2" pipe into a six inch pipe, and no water is being used, then the six inch pipe would measure 10 psi, just like the half inch pipe.

Jlisenbe,

Let's respect each other you don't have to say that Kiss is 100% incorrect he is very expert and knowledgeable person, he may have his own point, and you could have asked him differently, Tanks.

Regards,
John

OK, lets say you had a vessel that had no air in it, well acually 1e-6 torr and you had these flanges on this unit, some 8", some 1" etc. What's the force on the flange. Believe me the 8" flange turns out to be about 800 lbs and it takes lots of bolts.

When you say
What is considered normal water pressure? There really is no such thing as "normal" water pressure. Since water pressure is influenced by the elevation of the reservoir that serves your neighborhood versus the elevation of your water meter, water pressure varies from property to property. Water pressure in the District typically ranges from 45 - 125 psi; however, typical residential systems are designed to function best under a pressure of 40 - 60 psi. You are correct.
That's why city water towers are 100 feet high. That gives you a head pressure of 43.4 PSI at ground level.
Unless pressured up to reach outlaying areas the typical house pressure's around 45 PSI.
Jlisenbe, You could have worded your disagreement with KISS a little differently. A simple, "I disagree and here's why" would do. I'm sure that you didn't mean any disrespect but you could have put it a luttle nicer.
Play nice children! Tom

No disrespect intended! I can be a little too direct, so my apologies. My only point is this: The size of the pipe does not affect the pressure as measured by a pressure gauge. So, in a system with let's say 50 psi, the force of water in an 8" pipe is much greater than in a 1/2" pipe since the surface area as measured in square inches is many times greater. But the pressure in both pipes, when measured with a gauge, will still be 50 psi.

I agree with the above. KSS: I think your original explanation was mostly incorrect. Firstly 3/4 inch diameter pipe does not have 1 Square inch of cross sectional area so your calculation to get 10PSI doesn't work. Also when you talk about the 1/2 inch pipe pressure is measured over any area not just the cross section of the pipe i.e.. It could be measured over some surface area of the side wall of the pipe. A pressure gauge has a known small internal surface area that it measures the force on and displays it as a pressure (eg in PSI).

Certainly a greater diameter piston will have more area and thus any pressure in it will translate into a greater force on the piston.

Firstly 3/4 inch diameter pipe does not have 1 Square inch of cross sectional area so your calculation to get 10PSI doesn't work.

Never said it was 10 lbs. I said a 10 lb weight was applied.