Closed loop water circulation at superheated temperatures

RickDelta

Hello Heating Help Community! : )

I have a 100ft test "loop" consisting of 2" ridged PVC pipe with an in-line electric circulation heater and a variable speed water circulation pump (5' max. loop head).
The PVC pipe loop is 100% below the pump and heater.
This loop also has an "auto fill" pressure regulating valve water source and expansion tank.

Basically,
This test set up is to "balloon" this 2" ID PVC pipe to 4" ID ...... "inside" an existing 4" cast iron sewer lateral pipe.

The required water temperature is 220 degrees F held at 10 psi maximum water pressure.
(The 10 psi keeps the superheated water from flashing to steam)

At these temperatures, the 2" pipe can now be "ballooned" up to the new 4" dimensions by increasing the water pressure to 18 to 20 psi.

The problem i'm experiencing is that even though i'm holding the entire loop pressure above the flash point , I get severe water cavitation at the circulator pump when running at the required flow rate of 8 gals/pr min! : (

In my laymen's research, it appears that the centrifugal design of the circulator pump itself creates a localized "pressure drop" area right at the intake of the pump ..... but not throughout the entire water loop.
This "pressure drop" created apparently is lower than the required pressure to keep the water from cavitating.

Its my understanding, you would need to be at around 25% higher system loop pressures to offset this "localized" pressure drop to negate the ability of the water to cavitate.

The desired pipe diameter expansion (ballooning) process will not allow this higher, over pressurization during its polymer reforming (MO-PVC*) stage.

I'm thinking that I need to circulate this superheated water with a very low "pressure differential" type of a pump.

A "piston pump" looks to be the best option. I would think since were literally "pushing" the water around the loop hydraulically, there would be little or zero pressure differential in the loop (excluding the pipe loop constrictions itself of course.

Any pump experts here to advise?

Thanks!

* molecular oriented -PVC

https://molecor.com/en/what-molecularly-oriented-pvc

Jamie Hall

Your basic understanding is correct -- there is, inevitably, a low pressure region in and near the "eye" of all centrifugal type pumps. How low that pressure is -- and hence the static pressure at the inlet at various temperatures -- varies with the pump design, but none that I know of which are easily available will handle your combination -- or anything near it.

At first glance a piston pump sounds attractive. However. Unless you have a large accumulator in the circuit, or are using a pump with multiple pistons, the pressure will pulsate -- and in a shortish loop (say a few hundred feet) -- it will drop to zero; if it's a double acting piston, twice per stroke. If it's single acting, during the entire return stroke. This pulsation may or may not be tolerable in your application.

RickDelta

Thank you Jamie!

My thoughts would be to build a four cylinder, bi-directional, uni-flow pump under a PLC control. (w/ slow ramp up and interleaved pump strokes to quell the pulsations)

my question remains, ..... would this type of pump afford a zero or insignificant pump differentials?

RickDelta

leonz

Basic hydraulic flow is against you and you cannot heat water to a superheated temperature of 1,001 degrees without consequences unless you deaerate it.

All you are going to have of one section of burst pipe and accomplished what??????????

You need to learn more about reciprocating triple plunger pumps used in car washes and drilling mud pumps.

RickDelta

"You are going to have of one section of burst pipe and accomplished what??????????" - Leonz

......... I dont know what your stating here : (

RickDelta

leonz said:

Basic hydraulic flow is against you and you cannot heat water to a superheated temperature of 1,001 degrees without consequences unless you deaerate it.

All you are going to have of one section of burst pipe and accomplished what??????????

You need to learn more about reciprocating triple plunger pumps used in car washes and drilling mud pumps.

The required water temperature is 220 degrees F ..... NOT 1,001 degrees!!

RickDelta

"Basic hydraulic flow is against you" - Leonz

..... can you explain that in some detail please?

Jamie Hall

To go back to your questions, @RickDelta , a pump set up as your diagram shows will not reduce the pulsations. To do that, you would need two double acting cylinders as you have shown, connected 90 degrees out of phase. That arrangement would substantially reduce the pulsation although not eliminate it completely.

Whether or not it would reduce the problem of cavitation at the pump inlets, particularly in the area of the inlet valves, would depend entirely on the design of the valves and the exact inlet configuration. Without that information, I can only comment that, in general, very quick opening valves (either poppet or slide) with a waterway area substantially the same as the attached piping, together will all long radius flow paths, will reduce the problem, possibly to manageable levels. I doubt that check valves -- though attractive dues to their simplicity -- would probably not suffice..

Matt_67

You might check out Moyno progressive cavity pumps.

leonz

You are planning on heating water past the boiling point without a steam chest for one thing and the rubber bladder in the expansion tank may very well melt on you.

You need to read more about basic hydraulics with piston pumps. That is why I said you need to read more about in line triple plunger car wash and drilling mud pumps which can be single piston, dual opposing pistons, 2 cylinder piston plunger pumps triple piston plunger pumps and triple piston plunger car wash pumps.

RickDelta

@ Jamie
I was thinking spring loaded checks would be fine for the pump design shown above. That would be only one of 4 twin cylinder pumps in this system operating in staggered fashion.
The staggered piston strokes would be managed by the PLC (Programable Logic Controller)

RickDelta

@ Mat67:
For the flow rates required, their high rate of centrifugal spinning would also create inlet cavitation.
..... whereas piston design doesn't spin, just push!

RickDelta

@LeonZ:
"You are planning on heating water past the boiling point without a steam chest for one thing" - Leonz

NO ! I am NOT heating water past its boiling point! Look up "steam table" for temps and pressures im working with.

RickDelta

@LeonZ:
"rubber bladder in the expansion tank may very well melt on you." -Leonz

The piston seals, piston shaft seals and expansion tank bladder ... all made out of high temp "Viton" (450 degrees F)

Matt_67

Just curious if you actually talked to Moyno?  I’m not an expert on them at all but I do know they are a positive displacement pump.

RickDelta

@Matt_67:
No I haven't , but have read up on its operational principals and they graphically show you the dangerous cavitation in front of the "displacement cavities" (lobes).

RickDelta

hot_rod

I think a positive displacement pump also, a gear or vane pump. Like they use in hydraulics. 

RickDelta

Hi Hot_Rod!
Agreed! ...... even a set of "dosing" pumps!

The unique requirement here is to keep the lid (pressure) on the water from ever "flashing" ..... yet, not enough to excessively over pressurize the PVC pipe being slowly "morphed" into its new diameter.

hot_rod

What are the pipe lining companies currently using? It seems most of them expand a liner with heat and pressure. Are you looking at a higher temperature and pressure option.

RickDelta

@Hot_Rod:
I was was watching a pipe lining company do a sewer liner install. Its basically like this:
Think of a rolled up white athletic sock (fiberglass) impregnated with epoxy resin.
Put that into a special blower canister and blow (un-ravel) into the underground sewer lateral to be repaired.
Insert a rubber bladder and inject live steam to cure the epoxy resin.

What was funny about all this was that they advertise as a "No Dig" or No Trench" repair/installation ....... yet, the first thing that shows up at your home is a back hoe!! : )

Their system can't "blow" their new liner around/through the "sewer trap" in the front yard. So, they have to first dig down around 5ft, a 4ft x 4ft work area to remove the underground trap and only then be able feed liner directly into the straight run of the sewer lateral.
They had a crew of 8 men in all, as all the steps in the prosses was very time sensitive because of the preparation and working time of the mixed epoxy.

This got me to think, .... is there a better way to do this?

My design (prototype) takes a "ridgid" 2" PVC pipe and insert it from the lawn sewer vent cap (out on the front yard) down thru and around the trap all the way out to the city main sewer line.
After this 2" PVC liner pipe is in place, expand its diameter to the surrounding 4" existing sewer lateral pipe.
NO Digging at all!!
Crew: one plumber and a helper!
Less than halve the cost to the home owner.

OilfieldHippie

Why not just heat the water after the pump? 

RickDelta

...... I use no epoxy resin (very expensive) for my liner's strength.
I am "morphing" the 2" PVC liner to snuggly fit the 4" existing sewer liner by "molecular orienting" the PVC polymer with just superheated water and 20 PSI pressure.

Molecularly Oriented PVC is the result of reorganizing the amorphous structure of PVC-U into a layered structured, by a mechanical process, to provide unbeatable mechanical properties in a plastic pipe, such as: high impact resistance (almost unbreakable), high stiffness and fatigue resistance, excellent behaviour with external loads, elimination of crack propagation and maximum ductility.

This reorganisation or alignment is done through an elongation of the plastic in special conditions that allow the molecules to rotate following the direction of the deformation. Although these molecular changes are not visible at macroscopic scale, it is fully appreciable with an electronic microscope. As it is a physical process, there is no change chemically, so, there is no significant change to the PVC formulation.

RickDelta

@OilFieldHippi:
Hello!
"Why not just heat the water after the pump? " - OilFieldHippi

I don't understand your suggestion, please explain more in detail

leonz

You might as well subscribe to Trenchless Technology Magazine as there is a great deal more to pipe re-lining than simply remolding a pipe replacement.

Pipe has to be inspected, cleaned with various methods and machines washed out, re-inspected with a camera robot then the fun starts with pulling a coated felt liner through a pipe and inflating it then it is either cured with steam ultraviolet light using a 5 wheel pentagonal curing lamp cart the is expanded to the inner diameter of the pipe to be relined that is pushed or pulled through the expanded liner and curing the pipe with light just like a dentist does with some tooth veneer fillings.

OilfieldHippie

You’re worried about cavitation in the pump because of fluid temperature. Why not heat the water after the pump so the phase diagram is in your favor?

Positive displacement pumps will cavitate on the suction stroke even given plenty of boost from a c pump. We use triplex and quintuplex pumps every day in the oil field, and they will cavitate on suction even with 70psi on the inlet and cool water as the pumped fluid. If the fluid can’t fill the chamber as a liquid, it will do it as a gas. If you are trying to pump superheated water your window is tight and your pump rate would have to be super low to get the piston speed slow enough. 

Anyway I think you can do it with a pressure washer triplex pump and a heater after the outlet. 

EdTheHeaterMan

As long as the inlet of the pump remains at 10 PSI Gauge pressure, then the water will not boil until it gets close to 250°F. So at 220°F you should be able to get almost any pump to operate without cavitation.

Is there something on the inlet of the pump that will lower the pressure below 4 PSI when the inlet pressure is maintained at 10 PSI. Am I missing something?

RickDelta

"You might as well subscribe to Trenchless Technology Magazine" - Leonz

Long time subscriber to TTM!! ..... I get it sent hard copy! : )

leonz

SO DO I.

Jamie Hall

That's not a bad description of how it's done, @RickDelta . Except it's woefully incomplete. @leonz added some more of the critical bits to the story.

It's actually rather rare for the authority owning the sewer system to do anything to the laterals. That's the property owner's concern.

RickDelta

@EdTheHeaterMan:
Hi Ed! : )

"Is there something on the inlet of the pump that will lower the pressure below 4 PSI when the inlet pressure is maintained at 10 PSI. Am I missing something?" - EdTheHeaterMan

The 10 PSI is only a real pressure in the 100ft "loop" of superheated water,
but at the inlet of a centrifugal type pump the effective pressure is lower.
Hence, 10 PSI is not "seen" at the inlet of such a pump.

hot_rod

If you stretch 2" to a 4" diameter, the wall must thin? What wall thickness do you end up with? Is it brittle like typical PVC after it is over-heated?

EBEBRATT-Ed

@RickDelta

Just some ideas.

Most centrifugal pumps in a closed loop us a suction diffuser to straighten the flow as the water goes into the pump suction to eliminate cavitation.

This problem sometimes come up with boiler feed pumps that pump hot water from a boiler feed tank that is at atmospheric pressure and little or no "Head" on the pump suction the water cavitates. I know B & G used to make "low inlet head" pumps for that purpose.

Your pump at the top of the loop is also suffering from a "low inlet head" condition. Would it be possible to move the pump to the bottom of the loop?

Your flow of 8gpm through 2" PVC is very low.

I don't know how your are venting or bleeding the air from the system but that low of a flow will not move air . If there are any place air can trap in the system, it will stay there.

Just food for thought don't know if any of this applies.

RickDelta

@Jamie:
"It's actually rather rare for the authority owning the sewer system to do anything to the laterals. That's the property owner's concern." - Jamie

I never said they did! ..... Its the home owners responsibility to repair a damaged sewer lateral.

EdTheHeaterMan

RickDelta said:

@EdTheHeaterMan:
Hi Ed! : )

"Is there something on the inlet of the pump that will lower the pressure below 4 PSI when the inlet pressure is maintained at 10 PSI. Am I missing something?" - EdTheHeaterMan

The 10 PSI is only a real pressure in the 100ft "loop" of superheated water,
but at the inlet of a centrifugal type pump the effective pressure is lower.
Hence, 10 PSI is not "seen" at the inlet of such a pump.

Then make the inlet pressure 10 PSI so the pump will increase the pressure on the outlet side. Don't depend on the outlet pressure to stay at 10 PSI. By making the expansion tank location at the inlet of the pump and the water pressure source at the inlet of the pump, you can get your needed result with economical standard equipment. Most centrifugal pumps will operate at those temperatures. And from what I can tell, you are using the 10 to 20 PSI to expand the pipe with the high temperature water. Keep adding that water pressure at the inlet of the pump.

RickDelta

@EBEBRATT-ED:
"Your pump at the top of the loop is also suffering from a "low inlet head" condition. Would it be possible to move the pump to the bottom of the loop?" - EBEBRATT-ED

No! ..... The heated water "loop" is 5ft underground!!


"I don't know how your are venting or bleeding the air from the system but that low of a flow will not move air . If there are any place air can trap in the system, it will stay there." - EBEBRATT-ED

Air Bleeding/venting has been addressed in its design.

"Your flow of 8gpm through 2" PVC is very low." -EBEBRATT-ED

The 8gpm flow rate gives me a very satisfactory system Delta - T

RickDelta

@IolFieldHippi:


"You’re worried about cavitation in the pump because of fluid temperature. Why not heat the water after the pump so the phase diagram is in your favor?" -OilFieldHippie

I'm not worried about the fluid temperature at the pump! ..... its the resulting pressure drop at the pump!
A pressure drop lower than my "cap pressure" to keep my prosses temperature from flashing to steam!
This would result in a steam flash explosion at these temperatures and pressures!! : (

RickDelta

@Hot-Rod:

"If you stretch 2" to a 4" diameter, the wall must thin? What wall thickness do you end up with? " -Hot_Rod

I'm using schedule 40 , but could use schedule 80 if I desired a thicker liner wall.


"Is it brittle like typical PVC after it is over-heated?" - Hot_Rod

NO!!!! this change to MO-PVC gives it an awesome "hoop strength" and eliminates RCP (rapid crack propagation). You can utube MO-PVC and see bulldozers running over it and it just returns back in shape with no damage!


....... 100ft of "normal" rigid PVC pipe when heated at these temperatures will expand in length approximately by 18 inches!!
then contract in length by 18 inches when cooled.

When "morphing" the PVC this way (PVC to MO-PVC), there is NO pipe length or diameter contraction as we are holding the forming water pressure til after the liner is cooled. (The pipe polymers are "locked" to their new orientation)

RickDelta

@EBEBRATT-ED:
"Most centrifugal pumps in a closed loop us a suction diffuser to straighten the flow as the water goes into the pump suction to eliminate cavitation." -EBEBRATT-ED





I use "water straighteners" to quell the flow turbulence ahead of my turbine flow meters.

RickDelta

...... just a note:
I could use thermal fluid to eliminate all the flash and cavitation dangers (boiling points are over 350 degrees F) , but at a much higher system cost.
......... water from the customer's garden hose is free! : )