Pressure Control

Hi all,
There is a situation which has arisen right now at work. A pump has been chosen to supply water in a dorm. A pressure sensor is to be used on the distribution line. the sensor gives a 4-20mA signal. at 2 bar pressure when sensed, the pump should be stopped completely. and when taps are opened the pump is being controlled by an inverter which gives the pump required frequency to maintain the right flow. i am not sure how to make this situation go?? should i used a controller with PID progm or can i controlled it directly with the inverter(LENZE)?
The fact is that this situation would have been using a Booster pump since the cost is too high, i was told to perform this control whereby the cost is very effective indeed...

Can Anyone help Please,...

Hope to hear from you soon....

Taz...

You need to use an inverter with an external PID or setpoint controller.

The 4-20ma signal becomes your process feedback signal and you will need to input a setpoint which tells the drive what pressure to regulate to. This setpoint can be an analog input, or it can be directed to the keypad, or it can be an internal value in the parameters if there is no need to change it externally. If you are regulating water pressure in a building, the internal setpoint might be your best choice.

Usually, the drive will have a PID macro which configures most of the parameters, inputs, and outputs for this kind of operation. You will need to do some loop tuning where you set P, I, and maybe D gains. This is the part that requires a little expertise so you might need some help on the tuning.

Otherwise, it is mostly a matter of following the instruction manual's detailed instructions.

Thanks dick, this the right choice i think too. In fact 2day we discussed this matter in details. the Inverter being used will be a LENZE one. i have studied the manual of the inverter and so one pressure transducer will be chosen to give a 4-20mA signal and another pressure switch will be used to start or stop the pump. The inverter consist of On/Off control in its CONTROL terminal 28. It also have an analog input which can be configured to 4-20mA. along with there is also a PI enable within the Inverter. the real part will be to tune the P and I parameters. How should i proceed with, coz there will be no experts handling this situation only me and me...

I explain the situation:
There will be a pressure vessel which will be installed along with the pump on the line, whereby the vessel will prevent the pump from starting and stopping too frequently. so the pressure vessel will compensate a part of the pressure drop on the line.
So the pump should be controlled by varying its frequency according to the pressure on the line. we need to take in consideration also about the minimum frequwncy the pump does not give any flow as well. so i think a D parameter will not be neccesary!!!
Will the PI parameter be enough for this situation???
The real fact is to tune as you said....
Do you have any hints for me so that i can have a start please.. anyone...

Thanks

Taz...

look at this

this is a system that may do the job for you. It let's you send a flow and pressure command to it. It uses pump modeling and torque feedback to make it happen without transducers.
http://www.kadantunigy.com/

It's a little pricey, though..

I would assume that there is a standpipe or storage tank on the top of the building. If that is the case, you may be overcomplicating this. I would simply use the transmitter or even a simple pressure switch to turn the pump on when the standpipe level drops to a set value, say 1.9 bar indicated by pressure). When the pressure rises to 2 bar turn the pump off.

Since your system is mostly static head the VFD will save very little power and virtually no energy.

A VFD can only change pump flow, and you allude to pump flow in your question but you don't indicate any sensor for flow. That will make smooth control almost impossible. Everytime someone flushes a toilet your pump will cycle.

Hmm seems to be.......

The real objective is to maintain a pressure of 2 bar on the line. A booster pump works in the principle of a 3 or 2 stage pumps. i.e if a decrease in pressure is sensed by a transducer, one pump will be operating and if the pressure decreases to another set value the 2nd pump comes into action and so on.

What im trying to do is almost the same thing, but using only one pump. and i was asked not to control this pump the same way as booster as well i,e no set values for presure.

I need to control the pump continuously, since it is feeding about 72 bathrooms and a huge number of taps. on top this is a dormitory consisting about 500 people and we expect that they will all use water at the same time, since they will go to work and return from work at the same time.

About the storage tank, yes there is 3 water storage tajnk of 25 m^3 capacity each.
The aim of my customer is trying to save as much energy as possible.....

So how shall i proceed, any one can clear this situation for me..

Thanks...

Taz...

taz3m said:

Hmm seems to be.......

The real objective is to maintain a pressure of 2 bar on the line. A booster pump works in the principle of a 3 or 2 stage pumps. i.e if a decrease in pressure is sensed by a transducer, one pump will be operating and if the pressure decreases to another set value the 2nd pump comes into action and so on.

What im trying to do is almost the same thing, but using only one pump. and i was asked not to control this pump the same way as booster as well i,e no set values for presure.

I need to control the pump continuously, since it is feeding about 72 bathrooms and a huge number of taps. on top this is a dormitory consisting about 500 people and we expect that they will all use water at the same time, since they will go to work and return from work at the same time.

About the storage tank, yes there is 3 water storage tajnk of 25 m^3 capacity each.
The aim of my customer is trying to save as much energy as possible.....

So how shall i proceed, any one can clear this situation for me..

Thanks...

Taz...

Click to expand...

The problem as stated previous is you need to control both flow and pressure. In a solid system this is very hard.
NOW THEN
you said each tank is 25 M3 each that equates to 2500 liter which equates to 2500 kg per tank - pretty heavy to place at top of building without reinforcing structure to carry the load.

IF you can air load the tanks to the desired pressure at some value for headspace in tank then you could do with pump controlled by VFD using a level signal. The air loading would take care of minor surges ie one toilet flush and would allow the VFD and pump motor to have some uhhh "catch up pressure differential" I guess you would call it.

500 person dorm? I dont think all would be going to class at same time or is this a workplace dorm where they all start and stop at same time. Much easier to give em flex time so they stagger out the demand on the water system.

Dan Bentler

Your problem isn't when all 500 are showering at once, provided your pump has the capacity. If you try to control on pressure only with tight control then you will have a problem when only one or two are showering - your pump won't go low enough on flow.

The purpose of the storage tanks is to dampen demand variations. When water is used the level and pressure will drop, and then the pump will kick on to refill the tanks. You are essentially using the pressure transmitter as a level indicator.

There aren't any energy savings from a VFD on this system. If the pump is selected to operate at BEP (Best Efficiency Point) at the nominal pressure then slowing it down will drop pump efficiency and increase power. Plus there are efficiency losses in the VFD itself. It takes a specific amount of energy to lift a gallon of water to a given height, period. The only variable is the pump efficiency.

Hmm, yep i do understand now,.. so i need to opt for the simple on/off control as stated previously.. as with the VFD the pump efficiency which will be controlled...

when you say that if 1 or 2 people flushes, the pump will start to cycle.. but the fact that there is a pressure vessel of 210 litres on the line,.. dont you think that the latter will compensate for that....???????

So i shall be choosing this simple on off control then.. as you said why complicating this situation.

One last request from you guys,, please.. can you tel me clearly the consequences and the actions on the real system if im using a VFD for this control...

Please, coz i need to explain to my superior why im not doing the control he asked to...
Please....

Thanks a lot..

Taz...

hi again, .. i saw something when reading your post again, in fact the pump im controlling is not being used to refill any tanks, but are found at the end of the tanks line which will be used to supply the water to all the taps and showers etc.. in the dorm. on top the building is 3 storeys, the ground and first floor will be fed by gravity and only the 2nd and 3rd floor will be pressurised by the pump.
What im trying to say, is that this pump just do the final delivery to all the taps, etc on pressure. so there aint be any level control as far as i can see.. i dont know but if im mistaken, do tel me...

Thanks..

Taz..

Danesh

taz3m said:

i was told to perform this control whereby the cost is very effective indeed...

Click to expand...

In that case the system should be done by now.

What im trying to say, is that this pump just do the final delivery to all the taps, etc on pressure. so there aint be any level control as far as i can see.. i dont know but if im mistaken, do tel me...

Click to expand...

The story keeps on changing. It is no wonder you aren't done yet. I liked the idea of the pumps filling the tank at the top of the building using simple on-off level or pressure control and the hot and cold water mixing is done at the faucet as suggested in an earlier thread. Simple.

Note, you still need to recirculate the water from the solar panels through the hot water tank when there is low or no demand. This keeps the hot water tank from cooling off.

You are very confused, I'm afraid.

Draw a complete diagram of the entire water distribution system. Only after the physical system is understood can you develop the controls properly.

I intended to indicate that the pump couldn't go low enough if you had pressure and piping only without the tanks. You are correct that if you have the tanks it will dampen out the fluctuation from a minor change in flow. That is what I stated "The purpose of the storage tanks is to dampen demand variations."

Right now,its ok...
Hmm there is a drawing which i have posted long on the thread PID Problem.. this will help in understanding the water distribution.. in fact this is an automated solar water heating system... as stated by peter..

About the fact the water will need to circulate through the panels when demand is low, there is a control which recirculate the Hot water tank if its temperature is lowered from a set point.

Thanks..