A question about pumping water and modulating pressure- what pump?

Scenario is I need to make a rain effect for filming – quite straight forward in terms of rigging a leaky pipe overhead an pumping water to it – but I want to be able to modulate to quantity of rain as the shot is the first rain drops on water as it starts to rain – not a constant downpour. So not just turn on/off I need to be able vary the quantity of water travelling to the rain bar.

So I’d want to fit a tap / valve between the pump and rain bar – but what kind of pump (centrifugal, diaphragm, etc) could cope with the pump running but the water exiting the pump being restricted?

(supplimentary question – what kind of deal do I have to do with either God or the Devil to ensure its not going to just be pissing down anyway 🙂 )

To modulate the flow of water in a system using a valve you’ll need a centrifugal pump rather than a positive displacement one. You can make it work with a PD pump but you’d need to vary the speed of the pump itself which is probably more hassle than it’s worth.

Why not fit a bypass, would give you a bit more control and prevent surges when you open the valve.

Aha – never occurred to me to divert the water instead of restricting it (i’ve just woken up after two weeks of shooting so a bit dazed)

So I guess I go with centrifugal pump anyway and either a valve to split the flow, or split the flow and put a tap on the end of the diverted water – so restricting the bypass sends pressure up to the rain bar.

Sorted

have more than one rain bar?

header tank, hose + foot?

just saying is there not a simpler solution?

Centrifugal pump – preferably with a variable pressure setting on it so you can control what pressure it comes out at – can even used a sealed [pressurised] system with an expansion tank to avoid effects of cycling and guarantee the effect will look exactly as during rehearsal

Agree gravity and header tank will do the job as well.

Centrifugal pump with a valve, don’t forget the valve will restrict pressure as well as flow so a single pipe will broaden the pattern as the flow increases.

3 piston pump (CAT pumps) with an inverter speed control. Use it for filming duties then take it home afterwards and use it as a pressure washer

header tank, hose + foot?

We’ll be in the middle of nowhere and the rain bar needs to be about 5m off the ground so don’t really have a means of getting a header tank above it

that won’t help then unless your at the bottom of a steep valley…

you could always use a engine driven pump set from a tool hire place (guessing that you wont have mains water?) + you could get a bowser as well

yep – a IBC and 1000 litres in the back of the van – hired pump (either petrol or elec and a genny) – its control over starting the rain on cue and modulation so that theres a gentle start rather it being like someones set off the fire alarm 🙂

should have quite a lot of control with a petrol pump, I’ve use some fire pumps before and needed to top up a water tower and keep it full to the brim and maintain the level against the losses in a system with 16″ pipes and several miles of 50yr old pipes…..

recommend talking to Stuart pumps (worth checking out their used kit that’s for sale as well)

You probably want a centrifugal pump, a magnetic drive will give flexibility because as pressure approaches shut-in the motor will disengage from the impeller, preventing anything burning out or blowing seals. Without magnetic drive you’ll be limited in how long you can run the pump against a closed valve before something dies (unless you go to the complexity/expense of bypasses).

On the outlet you’ll want a valve of some sort (never close a valve on the inlet to a pump), broadly 2 types, ball, gate, etc are good for on/off but not so good for fine control (you’ll get 80% of the flow at 20% open), but you can still modulate them if your careful and would likely be fine. Valves for controlling are far more linear, and usually globe valves which force the fluid to change direction, pass through an orifice and a plug above the orifice effectively controls it’s size. You can get near linear control then, this is how taps work.

What size pipe from the pump ?
You need simple flow reg.
I could probably sort you something.
I simple tap or ball valve may not give you the modulation you need.

Hire a centrifugal pump from HSS and buy a Saunders diaphragm valve from Sean at VIP
http://www.vip-ltd.co.uk/

http://www.cranecpe.com/chem-energy/products/valves/industrial-diaphragm-valves/saunders-unlined-/-alloy-industrial-diaphragm-valves&page=09B3392B-9E42-6321-B7874CE81E1D4B33&brandid=6FE61C71-A9CB-D852-DEADDB2EB60F3284

Dont run the centrifugal pump for too long dead headed (closed outlet) as the water will get a bit warm being constantly churned. The diaphragm valve gives a nice smooth proportional output.

Plan is… a petrol driven 1″ centrugal pump with the out feed split into two hoses – a tap / valve on each allowing water to either be directed up to the rain bar or sent back round to the IBC – just straight forward fiddling with the taps to shut off/ modulate the rain. The centrifugal aspect means if we cock up our tap twiddling the pump won’t immediately burst but the bypass will prevent overheating when the rain is off. (probably while we wait for real rain to stop 🙂 )

I’m not having to do the full weather effect – sheets of rain against the landscape – the shot is down onto the surface of a loch and the rain drops landing so don’t need huge volumes.

Inverter or speed controller might do the job , although It might dribble out of the first 3 or 4 holes on the spray bar and none of the others.
What about sequential pumps? Or depending on max flow required you might get away with 3 or 4 spray bars and solenoids to switch each on in turn .
Build from 22mm copper in long lengths , center punch then drill 3mm holes.
IME you wont get droplets though , just alot of wee streams from each bar.
What you need is pulses or depending on time and budget something like a car fuel injection set up.

put your bar with the holes pointing up, with low flow the water will dribble out and round the pipe forming a droplet on the underside, then drop like a rain drop off the bottom of the spray bar.
Then use solenoids or lever ball valves to bring more bars into play to increase rain from almost single drops to constant rain , for down pour I think you would need inverted solid cone 45′ spray heads

I think you’re over thinking this STM!