How to compensate distributed and localised head loss in a circuit

Head loss is a very important aspect of a hydraulic circuit. It can be summed up with the expression “energy dissipation”, with the words referring to all the forces that oppose the movement that a fluid is forced to make.

Considering the energy dissipation that forms in piping is fundamental for correct system sizing, so that the flow rate is always correct for the circuit and also at all the outflow points. Head loss can be compared to drops in pressure and to compensate, the correct size of the piping and the characteristics of the system components – from the pumps that move the fluid to the valves that guarantee control – must be evaluated.  In addition to functionality and comfort, there is another important aspect tied to head loss: sustainability. As the correlation between head loss and energy consumption is evident, trying to contain head loss is equivalent to saving energy and reducing the environmental impact of our system.

Difference between distributed head loss and localised head loss

To compensate head loss, all the different situations it occurs in must be considered.

There are essentially two types of head loss that occur in a hydraulic circuit: distributed and localised. The higher the distributed and localised head loss, the more energy must be provided to the fluid to guarantee system performance.

Distributed head loss is a loss of pressure caused by friction that the fluid encounters as it flows. The loss is directly proportional to the circuit length, and this is why we speak of distributed loss along the whole length of the piping. These drops in pressure are directly proportional to the fluid viscosity and speed. The more viscous a fluid is, the greater the friction between its layers as it moves; in the same way, if the speed is doubled, the head loss becomes four times greater because of the friction that is generated. Head loss is inversely proportional to the section of piping. Under the same flow rate conditions, if the pipe diameter becomes smaller, the head loss increases considerably after the speed is increased.

The piping length, its diameter, the roughness of its inner surface, the fluid viscosity and speed: these are all elements that influence the distributed head loss in a hydraulic circuit.

Localised or concentrated head loss is caused as a result of single circuit elements. It can be tied to system construction, for example if there are fittings, bends, junctions or section variations, or can depend on the flow of fluid inside components such as hydraulic valves. In all these cases resistance that causes energy dissipation, and as a result head loss, is generated. The equivalent length method can be used to calculate the concentrated head loss. In practice, the distributed head loss can be used to calculate the localised head loss by replacing the localised resistance with a length of straight pipe that gives the same head loss. This is an extremely simple method, making it perfect for very long networks where the localised head loss is minimum in relation to the distributed head loss. When the localised head loss can make the difference and a more precise calculation is needed, it is better to use the flow coefficient or Kv coefficient.

The Kv coefficient defines the water flow (between 5° and 40°), expressed in m³/h, that passes through a valve with a differential pressure (loss of pressure) of 1 bar. It relates the valve flow rate to the head loss, making it possible to size the valves according to the required flow rate and the maximum head loss permitted by the system.

Hydraulic valves with a high flow coefficient

The flow coefficient is used to identify the most suitable valve size for system requirements, but can also be used to compare the efficiency of different valves according to size. The growing sensitivity towards saving energy certainly directs technical-engineering efforts towards reducing head loss. This can be done by working on the valve materials and structure to reduce friction and maximise the space that the fluid can flow through.  The search for less head loss has been guiding Mondeo from the very beginning, starting from its choice to use stamped stainless steel. Their slim walls, the smoothness of their stainless steel, the studies done on component structure to guarantee wide flow surfaces, attention to detail, such as Tig welding without fillers to avoid inner friction caused by weld seams: these are just some of the strong points of Mondeo valves. This continually developing research led to very interesting results. We have calculated that installing a valve-filter system (e.g. Idrja valve + Linia filter) produced with these characteristics in a system gives a 20% saving in energy.