Maintaining pressure: Flow losses of industrial valves

According to a survey conducted by the Fraunhofer Institute for the EU Commission, 12.6 % of the entire energy consumed in the EU is used for driving pumps, with pumps in industry making up the lion's share. Modern variable speed high-efficiency motors and flow-optimised pump hydraulic systems offer enormous saving potentials. However, pumps that transport liquids in industrial plants always just form one part of larger overall systems. Such systems also especially comprise pipes and valves. The question is: Is this somewhere opportunities for increasing efficiency may be found – so pumps can be dimensioned smaller or run at a lower rotational speed? To provide a direct answer: Yes, it is. By reducing flow losses.

Strictly speaking, it is incorrect to assume that pumps use a lot of energy – the pumps themselves consume only a relatively small share. Quite the opposite: It is the pump's task to put energy into a system, in the form of the fluid's kinetic energy. In actual fact, the energy is only consumed in the system. Assuming that a pump has got an efficiency of up to 90 %, the actual energy consumed by the pump itself is only 10 % – the remaining energy is transferred to the system, where it is consumed. See the illustration.

When a fluid flows through a pipe, pressure losses are inevitable. First of all, they are caused by friction between the fluid and the pipe wall: The rougher the inside of the pipe, the higher the friction and, with that, the pressure loss. The material (e.g. metal, plastic or clay) as well as possible deposits and oxidation in the pipe play a role. Secondly, friction effects also develop within the fluid due to the fluid viscosity (internal friction). The internal friction effect increases with the flow velocity of the fluid. Plant designers can significantly influence the losses from flow velocity by using larger pipes. Note: The flow velocity is squared in the pressure loss calculation.

The main cause of flow and pressure losses, however, are vortices and flow separation. Simply put: Wherever fluid is hindered from continuing its even flow, energy is lost. A piping system is not made up of a single, straight pipe. Usually, such a piping system comprises several pipe bends (elbows), branches, reductions/increases in cross-section, measuring instruments, strainers and, naturally, valves (from Open/Closed valves to control valves). Each one of these components causes some pressure loss and therefore energy loss. 

These pressure losses are indicated by the loss coefficient ζ – zeta. This is a dimensionless unit for the pressure loss of a component (e.g. valve). The larger the zeta value of a valve, the higher the pressure loss. To help with the selection, valve manufacturers also provide the Kvs value, also known as flow coefficient, of the fully open valve. The flow coefficient is the flow rate of water at 20 degrees Celsius that causes a differential pressure of one bar in the valve.

When adding up all pressure losses in the different piping system elements, a system curve of the piping can be drawn. The system curve forms the basis for calculating the required head of the pump as a function of the flow rate. You can imagine that in very large systems with several hundreds of valves, many deflections and long pipes, large amounts of energy will be lost.

As the wall area within a valve is very small compared to the overall piping system, wall friction losses in a valve can generally be ignored. The lion's share of pressure loss (the resistance coefficient zeta ζ) is caused by vortex formations, flow separation and secondary flows. The flow resistance of valves primarily depends on how much the fluid is deflected when passing through the valve, whether the cross-section is reduced and whether vortices form at the edges.

Valves are categorised according to their design features into the basic types of globe, gate, butterfly, ball or plug and diaphragm valves, and according to the functions they perform, i.e. shut-off valves, safety valves, control valves and check valves or strainers. A look at the internal design of the different types quickly shows that their flow passages vary significantly.

As a full-range supplier for the valves sector we offer a broad range of different designs. Alongside globe valves, gate valves, butterfly valves, ball valves, diaphragm valves and control valves, KSB's valve product range also includes actuators, positioners and strainers. No matter how different our products are in design, they all benefit from our 150 years of experience and our mission to always develop the best solution for our customers. 

Especially in times of strongly increasing energy costs, efficiency is gaining more and more in significance – in complex industrial applications even more so than in building services. Pressure losses are less relevant in building services as the flow velocity is relatively low at 0.7 m/s. In industry, flow rates can reach up to 4 m/s, which explains the trend towards considering the entire system's energy consumption in this field.

Our engineers develop products with a focus on several points: Maximum operating reliability and longevity naturally take centre stage. At the same time, the valves are hydraulically optimised to the maximum extent possible. Our BOAX or ISORIA butterfly valves, for example, are fitted with particularly thin discs to keep resistance to a minimum. Being economically efficient is the third criterion our products have to meet. Not everything that is technically possible makes sense from a commercial point of view. This is where an optimum has to be found without compromising on quality.

KSB generally places a high value on offering products of the highest possible efficiency – no matter if they are giant motors for driving heavy-duty pumps or small butterfly valves. Our aim is to always use resources as sparingly as possible and leave the smallest possible CO₂ footprint.