Gravity Fed Pressures and Solenoid Valves

Gravity fed system pressures.

Solenoid valves are commonly used for many applications described as a gravity fed valves, from a header tank or generally a low pressure closed circuit system. The essence of gravity fed is based on the principle of gravity against height. Gravity is measurement of acceleration and the standard unit of which is 1 meter per second squared (there are small changes or anomalies according to wear you are on planet earth), but suffice to say it is generally agreed to be 9.81 m/s². 

So as gravity is a fixed amount, the differences in gravity fed system pressures are generated purely from the height of the fluid. The higher the fluid the higher the pressure generated, both are directly proportional to each other and are unaffected by pipe or tube diameter (care to be taken on pipe diameter according to flow requirements).  

The illustration on the left shows a U tube with fluid in it. If pressure is applied at P the fluid moves up the tube outlet. The difference between the two levels "H" is directly proportional to the pressure applied. The more pressure P increases, the more H will increase, so if you double P you will also double H. when using water this is what is called hydrostatic pressure (Hydro = water static = not moving).

Hydrostatic liquid pressure calculation.

Liquid pressure or hydrostatic pressure can be calculated using the following equation.

Pressure = Height of fluid x density x gravity.

Gravity is 9.81 m/s² - Height of fluid column in meters - Density of fluid in Kg/m³

The table below shows water pressure against height for gravity fed systems.

This is why we have water towers, these are incredible simple devices offering a consistent mains water pressure utilising the gravity feed system and reservoir. 

What is water pressure?

Water pressure is the force used to push water through pipes, taps, hoses and fountains and determines the flow or speed of water supply. The amount of pressure at the end of the mains water pipe or your tap depends generally on how high the water tower (20,000 to 30,000 gallons) or water reservoir is above the pipe outlet (gravity feed) and how much demand there is elsewhere. The height of your property will influence your mains water pressure as the higher you are the lower the mains water pressure, and visa versa the lower you are the higher the mains water pressure, however installed water meters prevent excess pressure. 

Helpful hint: you are not allowed to pump or suck more water from the mains water system, as this can cause a vacuum, water deprivation elsewhere, potentially suck in foul dirty drainage water into the clean water system or collapse plastic pipes not designed for vacuum.

UK mains water pressure is usually 2 to 4 bar (30 psi to 60 psi), if you have less than 1.5 bar seek advise before installing showers or other pressure dependant equipment.

Solenoid valves for mains water pressure.

Solenoid valves for mains water applications fall into what the industry call the general purpose range of solenoid valves and are typically brass construction with NBR seals, power to open fail-safe closed, various threaded port sizes 1/4", 3/8", 1/2", 3/4", 1", 11/4", 11/2", 2" and even 21/2" and 3" BSP thread. 

There are two main types of general purpose mains water solenoid valve or gravity fed solenoid valve, basic of which are described below.

Zero rated solenoid valve - opens and closes regardless of system pressure, sometimes referred to as a direct acting solenoid valve, assisted lift solenoid valve, hung diaphragm solenoid valve, kick pilot solenoid valve or forced pilot solenoid valve. 

Good points: No minimum pressure difference required ideally suited to low pressure gravity fed systems.

Bad Points: Generally more expensive, larger port sizes consume more power, maximum pressure restrictions

Servo assisted solenoid valve - utilises the system pressure to open and close the solenoid valve, sometimes referred to as a pressure assisted solenoid valve, pilot solenoid valve, floating diaphragm solenoid valve, floating piston solenoid valve.

Good points: Generally cheaper, larger sizes consume less power and cope better with high pressure applications and higher pressure gravity fed systems that are open to atmosphere such as tank refilling and drainage systems. 

Bad points: Require a minimum pressure difference between 0.3 to 0.5 bar to operate correctly.

Read more about zero rated and direct acting solenoid valves for gravity fed systems.

Read our FAQ section for more information and guidance.