Automation Washing Station

So the pressure is not being maintained when the pump stops?

Congratulations. There is a leak somewhere. The system is losing fluid and the pressure dropping off is a symptom of that phenomenon.

thanks for ur wise answer.

i can't see why you want to have the output closed, the pump stopped and maintain pressure.

If you want to wash then presumably you want jets of water...well run the pump!...
When you want the jets to stop..... simply ...stop the pump. You can have a pressure relief valve if you wish.

Del

Check valve at output of pump most pumps will not hold pressure.

Sir,

Please check whether input from pressure switch setting is correct. In PLC [ladder diagram] please check the interlock of pressure switch and motor. The motor must switch off when the pressure is 20 bar. If possible use analog inputs for pressure switch. Please check online the PLC diagram and also when pressure switch reaches 20 bar the input must become 1.

fundamentally, your initial test should not work even if you don't have a leak in the system. liquid is imcompressible. this is how the hydraulic system works. you must apply a force (no matter how small it is that required) to maintain the pressure in the constant volume system. therefore, as soon as you stop the motor, without any leaks, the small change in temperature alone is enough for the pressure to drop. on the other hand, if the temperature has not change yet, all it takes is a displacement of a few drops of water or the collapse of the air bubbles.

first, you should know that 20 bar (300#) is a lot of pressure. i am quite certain that you can take a lot of paint off with that pressure. but then, i don't know much about automatic washing station. also, be sure to check your design and calculate the total volume of the system and, then verify with your local jurisdiction for pressure vessel requirement that it is not required the installation of a pressure relief or a thermal relief of some sort.

in general, if you are trying to use the line pressure (sensing by the pressure tx) to regulate the drives to speed up or slow down the motor, it may and may not work for the following reasons. the liquid is imcompressible, meaning you cannot maintain a fixed pressure at 300# if there is no user. keep in mind, that double the speed of the motor (which will double the speed of the pump) will quadruple the power and the work done by the drive/motor/pump is directly proportional to the volume throughput. therefore, you may not have enough resolution and response on the drives to maintain the constant header pressure at low usage.

the pressure switch can be used as an auto shut-off to stop the pump under high pressure or you can use it as a auto start to start the pump under low pressure but you should not use one pressure switch with its deadband to autostart and autostop the pump due to reliability and safety.

if there is a drop in pressure the pump must starts. according to u one drop of water leak will decrese the pressure. if there is no leake did the pressure be constant.

you are talking about an ideal condition of isothermal compression, which you could never achieve, realistically. plus, the idea is to be able to deliver the wash water while the pressure remain constant.

If you settle for a little lower pressure, like less than 150psi (10bar), the cheappest and easiest way to accomplish this task, variable flow at constant head pressure, is to add a pressurized tank and an air compressor, which always comes with its own pressure set switch. the compressed air will maintain the water pressure. if you must deliver water at higher pressure, this set up is not economic because the air compressor will be costly due to multi-stages and intercooler.

with this configuration, both of the water pump and the air compressor are running intermittently, on demand only. and the fluctuation in the demand for wash water is not a concern.