Vaccume System Relief Valve

[bstern]

I am putting together a vacuum system using a 10HP Fuji pump. I am trying to understand how the relief valve works to protect the blower.
As I understand it, the relief valve is either completely open when triggered or shut.

ShopBot tells me that shutting off all valves will tell you your max vacuum. My question is this: When you shut off all valves you cut off all air to the pump. This seems to me would emediatly trigger the relief valve.

This would also happen when loading a new sheet of non porous material to cut.

Am I missing something here?

Thanks,
Bob

[knight_toolworks]

a vac pump works the least when it is shut off completly

[Brady Watson]

Bob,
The relief valve is just like the one on your air compressor. If the pressure goes over (or in this case under) a certain amount, it opens to atmosphere until it gets back to a safe rating.

-B

[cnc_works]

If this is a regen pump, the way I understand it is if vacuum is completely blocked the relief valve opens proportionately to allow outside air in so the pump does not starve for air. Most high vacuum pumps do not mind, but a regen pump does not like zero air movement.

Donn

[Brady Watson]

That's right, Donn. When the pump starts getting up past it's intended rate of suction, the pump starts cavitating, basically producing a surplus of heat since it is getting out of it's efficiency range. The valve opens to let in cool air to keep the vanes from heating up and expanding, causing them to touch down on the housing...NOT a good thing!

If the valve opens under normal conditions or 'chirps', the 1st thing to check is the filter. Make sure it is clean and free flowing. Then, if necessary adjust the valve so that it pulls max recommended vacuum without chirping. Some require you to completely remove the valve from the unit, while others let you adjust it with a deep socket and long extention.

-B

[bstern]

So I am wrong about the valve. It only opens enough to let the min amount of air that the pump needs, therefor generating the highest vac?
I was told the valve was either open completely or closed. This makes more sense.

Thanks,
Bob

[Brady Watson]

Bob,
Under normal conditions it should be completely closed. It is a safety valve to keep the pump from self-destructing and nothing more. It's just like a pressure relief valve on your air compressor...we don't want the tank exploding now do we?

-B

[bstern]

Brady, I understand the overall function of the valve. To put it more precisely, if I understand the posts: The valve functions linear (open only as much as needed) not logically (on- completely open/off - totally shut).

Is this true?

The reason I ask is the rep at Fuji seemed to indicate that their normal valve works logically. If this were true, with a good sealed system and a full non porous (melamine) sheet the valve would just open and effectively eliminate any hold down until air flow increased enough to close the valve.

Are there different types of relief valves?
Do the logical ones just oscillate on and off and effectively provide enough hold down.

I have my pump and am ready to order all part to build my system.

Thanks
Bob

PS: sorry if I'm not thinking to clear. Setting up a new shop for the first time. Installing new PRS, Dust system, compressor, edgebander,.... Dealing with brain dead contractors.

Hope to have power (2 weeks to run 3 phase) to my PRS today and start having fun.

[Brady Watson]

Bob,
The relief valve has nothing to do with the volume of air. It only has to do with negative pressure/vacuum at a specific level. This level is set to be a little bit higher than the max normal operating pressure, so that if something is wrong, it can open to save the pump from overheating and crashing. When this valve opens up or starts 'chirping' it typically is an indication that your lazy butt hasn't cleaned the filter in a few months!
The valve should hardly ever be open to the atmosphere. IF the valve is open or chirping, it is telling you that something is not right. It will only open if pressure (in this case negative pressure) exceeds the highest recommended operating pressure, or wherever the valve was calibrated to open.

In terms of you vac system, you should completely disregard the function of the blow off valve for the time being. Just read the instructions that come with your pump, set it & forget it. It is nothing more than a butterfly valve with a tensioned spring on it....in functionality it's just like the one on your hot water heater...or your pressure cooker...your air compressor...your radiator cap...your air conditioner...a wastegate on a turbocharged car...etc.

Just for clarity, there is also another valve in the system that is often referred in ShopBot terms as a '5th valve'. It's the 5th because a typical factory furnished ShopBot vac system comes with 4 valves (one for each zone). The 5th valve is manually opened BEFORE you turn the pump on to ease startup, and then closed after the pump is running and the appropriate valves to each zone are opened. It is also opened when you are ready to remove your parts from the table and swap in a fresh sheet of material. Typically you do not turn off the pump between runs! Leave it running & open the 5th valve (just another PVC valve) to the atmosphere to relieve pressure so that you can get your parts off, without starving the pump for air. When you have your new material on the machine, you close the 5th valve and the vacuum now pulls the material down and you're ready to go again. Most manufacturers warn against starting and stopping the pump more than 3X an hour because it is hard on the motor and impeller vanes due to heat and expansion.

-B

[sawkerf]

I've had my Bot since 2004 and this is the first time I've ever heard such a thing about excessive starting and stopping my pump. I was not supplied the 5th valve with my kit. In fact, I barely had any instructions, just a few por, black and white pictures and no text. I don't say all this to slam my peeps at SB, it just goes to show how little I know about my machine. I've been wanting to ask about the very, high pitched, shrill noise I hear sometimes when I turn on the pump. Is that what you mean by "Chirping"? Thanks, Kip

[bstern]

After completing my vac table last night and doing some quick testing I found the answer to my question. YES, when you put a full sheet of melamine on the table the relief valve opens!!!

I must now find a filter for the relief valve as it will be open on a regular basis. Any sugestions? It has a 2" threaded opening and I don't have much room for another $275 SOLBERG.

On first look the system is great. The only problem is that when I open the 5th valve, it does not totally release the table. I may have to re-plumb it to the 3" part of the piping.

Installing a vac gage today and starting Test cutting!

[Brady Watson]

Bob,
The relief valve should not be open under normal conditions. Find the manual for the pump/system and read how to adjust it to the proper set point.

-B

[bstern]

Brady,

It seems logical to me. When you put a piece of non porous material on the table and you have done a decent job of sealing your plenum and spoil board you effectively have turned off all air to the pump. This would result in the relief valve opening. This was my theory, now proven fact on my system.

Where would air get into a properly sealed system to meet the min demands of the pump and therefore not open the relief valve?

Bob

[Brady Watson]

Bob,
It's OK for the relief valve to be open, provided that the pump's gauge is showing a value less than the safe operating rating for the pump. If it is rated for 14" Hg and it is reading 16" Hg, the pump is cavitating and runs the risk of touching down...destroying the vanes (if regen). The important thing here is that the pump is not 'chirping'...if it is, you'll want to adjust the valve.

If your vacuum setup is that efficient, you will want to install a filter on the intake side of the relief valve. Dust and debris will get into the pump and hurt it.

You can read more via: http://www.shopbottools.com/files/SBG00284051220VacuumHoldDown.pdf if you haven't already. Congratulations on a tight system!

-B

[bstern]

thanks Brady,

It turns out I do have a very tight system. When I picked up my relief valve (They are made right down the street from me) I had is set to 90% of rated vacuum. It opens with a full sheet of melamine on it. The pump is still not getting very hot. I can hold my hand on it with no problem.

Make a tight system and now I have to buy another $175 filter!!

Oh well, on first test, its doing a fantastic job of holding even small pieces (2" X 12") so I am a very happy camper indeed!

After I get into production I will give a full review of the fuji 10hp pump. Its cost is not much more than 2 fein's. I think you can get them for about $1,100. The spec are 8in Hg with a max of 388 SCFM. I have not installed a vac gauge yet to see what I am actually getting.