# Adding a pressure switch to a compressor



## curtisrider (25 May 2017)

I acquired a compressor with a duff motor last year and have only just had a chance to take a peek at it. After replacing the motor with a spare it pumps a treat but there is a important component missing... the pressure switch! I have whacked on a regulator and it all works fine but I daren't leave it on too long for fear of ending up in pieces.

I'm not sure how it has been used without one, and to be honest I'm not entirely sure how to plumb one in as I'm not sure where I can tsp into so I was hoping you guys could point me in the right direction. The make is C&H who haven't existed for about 30 years it seems however I'm fairly confident something can be retrofitted. I'm not 100% sure of my terminology and what I need so if people could correct me or advise as to what i should be fitting/replacing that would be great.





At the back on the side is this which I'm assuming is a massive drain plug






At the back on the top, tank pressure gauge left, some sort of pressure valve (spring loaded inside) in the middle with a handle welded to it for a reason i'm not entirely sure of, and another pressure valve on the right





At the front on the bottom there is a drain tap that I don't understand how I'm supposed to operate





And at the front side is another drain area with a tap, from what I have gathered this may have been for a certain type of air tool to connect to?





I have been looking at pressure switches online and I'm not sure what is suitable, there seems to be generic cheap ones that vary from £6 to £30 yet seem to be the same unit, I also think I should replace the pressure release valve/s for peace of mind but I'm not sure how many bar they should be rated at?

Any help would be great, it would be nice to get the old thing running safely


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## dzj (25 May 2017)

In broad strokes, there are 2 types of pressure switches. One needs to be connected to the unloader valve, the other type doesn't. 
You'll also need a pressure release valve, usually one that reacts when pressure exceeds 9-10 bar, depending on your tank's max
prescribed pressure.


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## curtisrider (25 May 2017)

Hi dzj, thanks for your help. I've just searched unloader valves and I can't identify anything that looks like any parts on my machine, does that mean I want one that doesn't connect there?


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## dzj (25 May 2017)

If your compressor can't start under pressure there should be a check valve ( a kind of one way valve) where the red arrow points.
From this valve a small tube goes to the unloader valve in the pressure switch.
The small tube (blue arrow) I'm not sure what it's for (possibly for pressure release). 
Maybe someone with experiance with similar models can shed some light.

HTH


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## curtisrider (25 May 2017)

I've not had it under any significant pressure yet as I'm nervous to test it until I'm sure of the pressure relief valve working so I can't comment on starting under pressure.

The small blue pipe goes to what seems to be a pressure release valve but for some reason there is a handle welded to it


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## sunnybob (26 May 2017)

how old is it? and how good is the condition of the tank?
You should realise that you have a potential shrapnel bomb there.
Pressure vessels should be inspected and tested regularly. 
If there is an accident any insurance company will want to see an inspection certificate before paying out any claims, and if other people were involved you would be liable for payouts.
I used to have to strip down water boiler vessels just so the inspector could look inside and then I had to reassemble the things, and they were only working at 1 or 2 bar.

Not being an alarmist, but I would not be in the same room as an old untested compressor.


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## Myfordman (26 May 2017)

Make sure the turn on pressure is high enough to get correct operation of tools like nailers without having to set excessive off pressure. 
A switch with adjustable hysteresis is best but will cost more.


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## novocaine (26 May 2017)

I agree with Bob on this one, it's not a nice thing when one lets go violently. 

but if you are adamant you want to use it, remove the rather defunct hose connector (see below for what to do with it) and fit a new inline pressure switch, relief valve and regulator. you can see the unloader line coming from the head to the current (Rather dodgy looking) PRV reason for the welded on bar is most likely so you can lift the valve and release the pressure from the head which tells me it doesn't have an unloader valve, so guess you'll need one of them too. that will need to be routed to the new pressure switch. that valve is in the perfect place for the new stuff, which is lucky as I'd want rid of it anyway, it won't be in good shape. oh yer, the other little stubby thing, thats also a PRV.

so this for starters (yes it's one of your cheapy one's. not industrial and won't last forever but will do for starters)
https://www.ebay.co.uk/p/?iid=252689643 ... t=0&chn=ps

you are going to need a bag of fixtures and fittings too (which can and does get expensive) and don't overtighten anything going in to the switch, it's all tapered thread and it will crack and split on you (aluminium/zinc castings), you've been warned, it's not the fault of the fitting. use PTFE tape.

that "drain valve" you say at the front, isn't a drain valve thats the main outlet at the moment (can't drain anything when it's on the side of the tank), remove it and fit a plug, you don't need it. basically, if it isn't on the bottom of the tank, it's an outlet not a drain plug. 

the little valve is the water drain valve, very very important and the reason why most old compressors get chucked, because when they aren't used the tank corrodes on the inside, which is what the big plug on the back side of the tank is for, inspection. 

as to what pressure you'll need. it's on the plate a the front. whatever it says, half it, no way is that tank suitable for full pressure anymore. 

seriously bud, if you aren't sure about it, don't use it, whilst it may go with a pop and fizz there is a large chance it'll go with a bang and send an end cap across the workshop.


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## DennisCA (26 May 2017)

The motor and pump can be used though and hooked up to a new tank. I have a small 35 year old compressor with a 25l tank. I have gotten a new (old) 96 liter tank in excellent condition, actually from a cooling unit and is rated for 26 bars of pressure. That kind of marginal makes me feel safe. Since I keep to around 6 bars now, 8 with this new one. 

I recommend to set up a good drainage system, I will use a ball walve with hose so I can just open it a little to blow out water that accumulates.


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## sunnybob (26 May 2017)

Dennis, it WAS rated 26 bars when it was new. How many years has the water been sitting inside it slowly eating away the metal? Check the label and see when it passed out of spec.

i might be banging a drum here, but I have one story that I can testify to being true. i spent 6 years in charge of LPG storage tanks on military bases, anything from 500 litres to a whopping 15,000 litres of liquid gas. 5000 litres was the norm.
As a matter of course, those tanks were rated for 10 years and were swapped out every ten years regardless (hugely expensive operation).
In between times, an independant insurance inspector came and checked those tanks every two years.

I was with him one day and he had a sonic gizmo that could read the steel thickness just by placing the sensor on the outside. 
He found one tank that was only 4 years old had a spot where the metal was only half the required thickness.
That tank was taken out of commission and changed. It wasnt even half way through its "rated" lifespan.

Those tanks were rated for 17.5 bar, but very rarely were at more than 7 bar, and usually between 2 and 5 bar. 

Dont take chances with containers under pressure, it's the INSIDE condition thats important, not the surface rust.


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## curtisrider (26 May 2017)

Hi chaps,

Thank you very much for all your help, it has been quite a learning curve. It looks like from the general feedback that I should scrap it as I cannot verify how well it has been looked after so I'll take the motor off and put it on the heap, I'm very gutted as I can't afford another one and my other working compressor is in even worse external condition (it is drained regularly though) so that'll also need to be scrapped. Time to get saving the pennies....


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## sunnybob (26 May 2017)

I've given you the worst, heres a small crumb.
Remove all the fittings from the tank.
get a torch and a small mirror, look at the inside. If its rusty as %$"k then dump it. If its sparkling new like the day it was made, then just do it.


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## pcb1962 (26 May 2017)

sunnybob":qiahwdwz said:


> I've given you the worst, heres a small crumb.
> Remove all the fittings from the tank.
> get a torch and a small mirror, look at the inside. If its rusty as %$"k then dump it. If its sparkling new like the day it was made, then just do it.


I would also spend a few quid to hire a hydraulic pressure tester


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## DennisCA (26 May 2017)

sunnybob":3hzwf3s7 said:


> Dennis, it WAS rated 26 bars when it was new. How many years has the water been sitting inside it slowly eating away the metal?



0.


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## sunnybob (26 May 2017)

The compressor uses air, which has a water content. There are drains on the tanks to allow removal of the water. If the previous owner was lax or didnt bother at all, or it has sat for several years there could have been serious amounts of water inside the tank The tank is iron and damp air rusts iron out quicker than anything else.
Without a sonar thickness tester, that tank could be just as I described in the story above, despite the outside being bright as a new pin.


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## DennisCA (27 May 2017)

No, because you know what. I inspected it with a camera on a stalk before buying it, the inside is utterly spot free, not a speck of rust. And it's not been used as an air compressor.... I wrote that but I guess it's more fun to preach.


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## dzj (27 May 2017)

To use an old tank or not?
Everyone has heard horror stories, but also a lot of people use old compressors every day in their shop.
The tank you have looks sturdy on the outside. You've nothing to loose if you remove the fittings and have a
look on the inside. A little rust isn't a problem but look out for pitting and such, usually around the drain valve.
Ultimately, if you're uncomfortable with using it, then don't.
Or buy a new 'air pig' tank and use the motor and pump you already have.


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## sunnybob (27 May 2017)

dennis, apart from your attitude to a considered warning, I'm glad you had the foresight to inspect it before used it. Many people dont understand that. I was merely trying to prevent a serious accident.
I shall make a point not to preach at any of your future posts.


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## curtisrider (27 May 2017)

I have an endoscope so perhaps I will just open up one of the side outlets and have a peek inside before writing off both of my tanks. Thanks for the idea!


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## hawkeyefxr (1 Jun 2017)

You compressor looks very mych like mine though mine is a Halls compressor.
The drain at the bottom of the tank is to drain off water or maybe oil, which is my case. The oil from the compressor makes its way into the air tank. Air under pressure will make a small amount of water.
Halls are essentially an air conditioning company but have been around for many many years. I have an air/con HP/LP switch (high pressure/low pressure. The HP side governs when the compressor cuts out and the low pressure governs when it cuts back in as you use the air.
These are available from places like HRP air con supplies, you will need some small copper tubing and fittings (all air con stuff)
My compressor is older that yours and i run at a max of 85psi


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## LancsRick (1 Jun 2017)

I think I'm right in saying you can get compressor tanks hydro'd just like you would a diving tank. I certainly always keep mine in check (in fact the dive shop won't even fill them without a certificate, very sensibly). I realise the pressures are wildly different but it's still a thought if you want peace of mind.


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