Need help with some Pneumatic Terminology

[Matthew Wang]

Hey all,

This is my team's first year using pnematics. I was going over the rules and read this one:
[R71-C] Solenoid valves with a maximum ⅛ in. NPT port diameter, and a maximum Cv of 0.32 (if non-KOP valves are used, the team will be required to provide part documentation validating that the valves meet these constraints).

I'm wondering what both the NPT port diameter and the Cv mean. What do they do on the solenoid?


EDIT: Another question: How exactly do you calibrate the pressure relief valve?

Also, any other general tips on putting pnematics together?

[Mark McLeod]

Cv is a measure of the air flow rate.
It can be difficult to determine if the manufacturers specs use a similar but different measure.

NPT port diameter is an internal port inside the solenoid. You have to find this spelled out in the manufacturers specs too.

To set the pressure relief valve you put the system under pressure and turn the end until the air escapes at the proper psi.

We have some general information on pneumatic layouts and setup here:
http://www.team358.org/files/pneumatic/

[EricH]

Cv is a flow control spec when we're talking pneumatics. (MEs dealing with thermodynamics have another meaning for it.)

NPT port diameter is how big of a threaded connecter you can put in.

Taken together, it's how fast you can release air through a particular port size (and you can't use a larger port or faster release).

For calibrating the PRV, you'll need a gauge and the PRV attached to the robot. If you're already put together, you'll need to put a short across the pressure sensor, or disconnect it--anything to make the compressor run--and you'll need to be able to see the high-pressure gauge while you run the compressor. You run the compressor, but watch that pressure gauge. What it reads when the PRV hisses is the calibration setting. (If you hit 130, STOP and VENT immediately.)

If the PRV isn't set properly, you have to move it in or out. Move the small nut away from the end, then move the large end piece in or out on the threads. Check again. If you go the wrong way, go back the other way. Check again. Repeat until the PRV is calibrated properly. Lock the small nut against the large end piece.

[chadr03]

NPT port diameter refers to the tapped thread that you screw a pipe or tubing fitting in. There are many standard sizes 1/8, 1/4, 1/2.... The tricky part with them is that the tapped hole will actually be larger than the size listed. A 1/8" NPT tapped hole is around 3/8" in diameter. The 1/8" is the approximate inner diameter or the pipe that will thread into that hole. You can find tables of standard dimensions online or in a Machinery’s Handbook.

Cv is a flow factor or flow coefficient. It is based on the inner geometry of the valve and how well something can flow through it. It is used to calculate flow capacity of valves and such.

The pressure relief valves have come pre-calibrated in the past. I would hook it up to your system very near the compressor and try to pressurize up to 120 psi and see if it starts venting. The valve has a spring that basically closes an exhaust port. When the pressure inside the system overcomes the spring force the port will open. If your valve does not open then it can be adjusted by loosening the jam nut (thin nut) then by turning the top cap with the hole in the end. If I remember right clockwise will increase pressure, counterclockwise will reduce the pressure.

[Al Skierkiewicz]

Matthew,
NPT=National Pipe Thread.

[Mr V]

I told one of your mentors how to adjust the PRV on Sat, and I can show you in a few minutes as I'm heading your way now with the part we discussed.

[Gary Dillard]

Originally Posted by Al Skierkiewicz

Matthew,
NPT=National Pipe Thread.

Actually Al, I always thought that was what it stood for. But just now I was looking up some pipe thread dimensions for a job at work (yes some of us actually have a real job this time of year) and found in Mark's Handbook that the T stands for Taper. NPS would be National Pipe Straight.

[Nate Laverdure]

Originally Posted by Gary Dillard

NPS would be National Pipe Straight.

Nominal Pipe Size

[Gary Dillard]

Originally Posted by Nate Laverdure

Nominal Pipe Size

No, actually in this context it is National Pipe Straight as I said.

From ANSI B1.20.1 "Pipe Threads, General Purpose", Section 1.2 Thread Designations:

1.2.2 Each of these letters in the symbols has a definite significance as follows:
N = National (American) Standard
P = Pipe
T = Taper
c = coupling
S = Straight
M = Mechanical
L = Locknut
H = Hose Coupling
R = Railing Fittings

Not to be confused with nominal pipe size (NPS); pneumatics generally uses tubing, which is specified by OD, not NPS.

[Nate Laverdure]

Originally Posted by Gary Dillard

No, actually in this context it is National Pipe Straight as I said.
...

Ah, I see, thanks.

[Al Skierkiewicz]

Thanks Gary,
It would seem that some vendors are still using old definitions.

[FrankJ]

The problem with TLAs (three letter acronyms) is that they mean different things to different people (Or the same people at different times.)
Pipe terminology is particularly bad because of historical precedent from different regions.

NPS to a lot of people & some standards is nominal pipe size. If you told them you wanted straight pipe thread, they would just look at you funny. (If you doubt that go to your local hardware store an ask them, if you live in the USA)

Cv is the conductance of flow. (inverse of resistance.) It is a handy way of knowing how much a particular valve or fitting flows. The easiest example is with liquid. A valve with a Cv of 1 will flow 1 Gal/Min with a pressure drop of 1 PSI. Note the unit dependance. Cv with pneumatics is more difficult because the flow is referenced to standard conditions. (Cubic feet per min at 14.47 PSI at 32 F, or 60F or 68F or 70F). Lets not get started with metric definitions. Anyway Cvs determined with consistent standards with let you know the relative size without worrying about all the illogic & math behind it. Which is what standards are all about.

General tips.
Go to the rules. Make sure your pneumatics is laid out in the correct way & that you understand well enough to explain it to your inspector.

Use Teflon tape on tapered fittings. Make sure the tape ends before the threads do so that you do not get bits of tape in the system or block the end of the fitting. Do not over torque the fittings.

Do not use teflon tape on straight thread fittings. It will not do anything useful.

Make sure you tubing is cut square. It seals better in the push fittings that way.