Technical question (my lecturer couldn't answer it!)

[Doctor Drivel]

"Welcome, Drivel. Why have you chosen to discard your alter ego identity, Water Systems?"

Thank you Maxie. Maxie I am not Water Systems, I am Doctor Drivel. I don't know as much about certain aspects of life as Maxie - and never will.

same reason he no longer posts as "Adam", "MiniMe" and all the other nom de plumes

blown out I expect

I can't speak for Water Systems Maxie. I can speak for me though.

How was Malta Maxie? Are you still drunk?

 

[Spark123]

Is it illegal to have a pressure washer connected directly to the mains, or is it OK as it only has a low flow rate?

it is fine to have a pressure washer connected to the mains as long as it is connected via a double check valve bib tap.

Thanks, I'll have to look into buying one afaik my current bib tap doesn't have a double non-return in it, its been installed for years now. I take it I am allowed to install a check valve (or would it be two? can you get double?) before the tap instead?

 

[doitall]

AFAIK all pressure washer have to have an air break, a check valve is not acceptable.

 

[Doctor Drivel]

The idea is not to cause negative pressure in the mains. This can cause back siphionage of contaminated water from a house into the mains. Hose taps require double check valves inside them. If the hose pipe is in a dirty pond and there is back siphonage and no check valve contaminated water will enter the mains. The same with flexible shower hose too. Pretty simple.

See? Exactly what I'd said.
Dr. Drivel knew you wouldn't believe me, but now that HE has deigned to confirm it, surely no-one will dare to doubt it.

Welcome, Drivel. Why have you chosen to discard your alter ego identity, Water Systems?

Mr Tap, I am Doctor Drivel. You have been boozing it up with Maxie Raden tonight haven't you?

 

[doitall]

I think that many of you should be taking theWater Regulations course!

Onetap has explained some of the health reasons for pumping from the mains to be prohibited.

The simple answer is that it is not permitted. ( Thats a full stop! )

Tony

Wrong Tony, there are several approved pumps that connect direct to the mains giving a maximum 12l/min.

Remember the accumulator topic, they have one for starters.

 

[Doctor Drivel]

1. Anti-Vacuum Valves. T&PRVs do not in themselves function as AVVs, but may have an AVV incorporated into them. It seems that some manufacturers do not fit AVVs!

2. As onetap says, greater outlet flow than inlet flow on a closed system unvented cylinder would mean contraction (collapse) of cylinder volume. However the negative pressure required to collapse an unvented cylinder would be unlikely to be achievable in practice. The expansion vessel membrane would surely explode first, releasing air into the system.

The cylinder would implode when less that atmospheric pressure inside. If 3 bar and a drop to 2 bar the cylinder is still above atmospheric pressure. It is when the mains pressure drops that implosion may occur when running a pump on the draw-off.

 

[Onetap]

However the negative pressure required to collapse an unvented cylinder would be unlikely to be achievable in practice. The expansion vessel membrane would surely explode first, releasing air into the system.

I don't think it would take much negative pressure at all.
When it's under positive pressure, the walls of the cylinder are in tension. When under negative pressure, they're in compression.

I'm reluctant to try it with an unvented cylinder, but a plastic Coke bottle is a disposable pressure vessel you could try it with. You can blow a lot of pressure in without inflating it (much). You can suck it flat quite easily.

The only pressure vessels I can think of built to withstand a compressive force are submarines. They have lots of internal supports to bear the loads and aren't really thin cylinders. A hot water cylinder just isn't intended to take negative pressures and it won't.

 

[Spark123]

AFAIK all pressure washer have to have an air break, a check valve is not acceptable.

What is this air break you speak of? Like a header tank? Or is it to do with the double check valve bib tap?
Sorry for dragging this one out, I'm just interested on how things should be done.

 

[chrishutt]

A hot water cylinder just isn't intended to take negative pressures and it won't.

So why do some manufacturers not bother with AVVs? They obviously have confidence that the cylinder won't collapse that easily. And you can hardly compare a flimsy plastic bottle with an unvented cylinder with several mms thickness of steel wall .

The maximum possible pressure tending to collapse a cylinder is 1 bar, even if a perfect vacuum were achieved internally. A submarine on the other hand might be subjected to external pressures of say 10 bar at 100 metres depth. Again a spurious comparison.

 

[doitall]

AFAIK all pressure washer have to have an air break, a check valve is not acceptable.

What is this air break you speak of? Like a header tank? Or is it to do with the double check valve bib tap?
Sorry for dragging this one out, I'm just interested on how things should be done.

Pressure jet washers have a built in water tank.

AFAIK that is the only CE aproved design acceptable by the water Authority's

 

[Agile]

Chris and John, I said "the simple answer" ! Until Lorraine has taken her water regulations I am sure thats adequate for her!

I dont know about Bath, but in Thames Water area the max pumping allowed is 7 li/min. I thought it was the figure in the regulations!

I do think that "Dr Drivel" is a far better name for Water Systems because it so accurately describes what he posts !

He goes on holiday and comes back with a new identity!

Tony

 

[doitall]

A number of calorifiers in the industrial large domestic, Clyde for example, you have to fill the inner domestic tank before the outer primary water or the cylinder will collapse.

Should any of you come across such a beast and you have to drain out the domestic side then you must first drain out the primary's and for that reason they don't have a draincock, you have to induce syphonage.

I will read the rest and comment tomorrow.

 

[garethgas]

Been following this thread with great interest. I have nothiing to contribute in a technical way but what a refreshing change to see a civilised discussion without the 'I know more than you' brigade.
Been a gas engineer, lecturer (resent those comments lol) and now self employed ( 5 yrs) and I STILL learn so much when the conversatiion is in a civilised, aduld form.
Thanks to all you contributors.
watch Softass spoil it any minute!

 

[Onetap]

A hot water cylinder just isn't intended to take negative pressures and it won't.

So why do some manufacturers not bother with AVVs? They obviously have confidence that the cylinder won't collapse that easily. And you can hardly compare a flimsy plastic bottle with an unvented cylinder with several mms thickness of steel wall .

I think you're overestimating the strength of the cylinder and/or underestimating the power of atmospheric pressure.

It won't be several "mms" thick. If 1 mm were adequate to withstand the pressure and a safety factor, then they won't increase their costs by using anything thicker. A flimsy plastic bottle is a very good analogy for a flimsy stainless steel cylinder.

It's designed to withstand positive pressure. If you should choose to subject it to negative pressures or excessive positive pressures, then I'm afraid that's not covered by the guarantee, Sir, but we'll be pleased to sell you a replacement.

How about this for a cylinder collapse?

http://www.delta.edu/slime/cancrush.html

That is 7/16" (11mm) minimum thickness steel plate. It is pressure tested to 100 psi and had a burst pressure of 500 psi. It was accidentally subjected to something less than 14.7 psi negative pressure. And it didn't have a vacuum relief valve.

The maximum possible pressure tending to collapse a cylinder is 1 bar, even if a perfect vacuum were achieved internally. A submarine on the other hand might be subjected to external pressures of say 10 bar at 100 metres depth. Again a spurious comparison.

Not at all. The sub is designed to cope with 10 bar. The cylinder might be designed to cope with 10 bar internal pressure. It is not designed to tolerate any external pressure and it won't. They crush up like tin foil. See link above.

 

[Doctor Drivel]

Chris and John, I said "the simple answer" ! Until Lorraine has taken her water regulations I am sure thats adequate for her!

I dont know about Bath, but in Thames Water area the max pumping allowed is 7 li/min. I thought it was the figure in the regulations!

I do think that "Dr Drivel" is a far better name for Water Systems because it so accurately describes what he posts !

He goes on holiday and comes back with a new identity!

Tony

Water Systems can't have my name. Maxie Raden went on hols. You are confused. He went larger louting.