Control valves: choosing the characteristic, avoiding cavitation and noise

The characteristic describes how the Kv value changes with travel. With a linear characteristic every equal step in travel gives the same increase in Kv. With an equal-percentage characteristic every equal step gives the same percentage increase relative to the Kv value at that point – the curve starts flat and becomes steeper towards the top.

In figures, for a rangeability of 50:1: at half travel the linear valve has reached about 51 per cent of its Kvs value, the equal-percentage valve about 14 per cent. At three-quarters travel it is roughly 76 against 38 per cent. The equal-percentage valve controls small flows with a lot of travel and therefore with fine resolution.

Both curves apply under a condition that is rarely met in a plant: constant pressure drop across the valve. That is why they are called inherent characteristics.

As the valve opens, flow increases – and with it the pressure loss in the rest of the loop. Less pressure drop remains for the valve. Flow therefore grows more slowly than the inherent characteristic suggests. What actually results is the installed characteristic: flow against travel, in this system.

How far it departs from the inherent one depends on valve authority. At high authority the shape is largely preserved. At low authority a linear characteristic turns into one that releases almost the entire flow in the first few per cent of travel. Under the same conditions an equal-percentage characteristic is distorted towards linear – and so delivers in operation the behaviour wanted from the control loop.

The aim is a loop whose gain stays as constant as possible over the load range. The valve characteristic should compensate for what system and process distort.

At the narrowest cross-section of a valve velocity is highest and pressure lowest; downstream the pressure partly recovers. If it falls below the vapour pressure of the liquid at the narrowest point, vapour bubbles form. If it rises above the vapour pressure again downstream, the bubbles collapse abruptly. The resulting pressure pulses strike plug, seat and body wall.

Cavitation is recognised by a crackling noise, as if gravel were being conveyed through the line, by vibration and – on the dismantled valve – by rough, pitted surfaces. With severe cavitation the flow also stops increasing with pressure drop.

How susceptible a valve is depends on its pressure recovery. Designs with a streamlined flow path – ball valves, butterfly valves – recover pressure strongly and cavitate earlier than globe valves. Manufacturers state characteristic values for this: the pressure recovery factor F_L and a value for the pressure ratio at which cavitation begins.

If the pressure downstream of the valve is permanently below the vapour pressure, the bubbles no longer collapse. Part of the fluid leaves the valve as vapour, and the line downstream carries a mixture of liquid and vapour at high velocity. That is flashing.

The damage pattern is different: smooth surfaces that look washed out rather than pitted craters, caused by droplets striking at high velocity. Above all, flashing is not a matter of valve design. Whether it occurs is decided by outlet pressure and temperature – that is, by the system. A different valve can reduce the consequences, not remove the cause.

Against cavitation, anything helps that prevents the pressure from falling below the vapour pressure in a single step. Against flashing, it helps to place the vaporisation where it does no harm.

With liquids, cavitation is the main source of noise – avoiding it usually gives a quiet valve as well. With gases and steam, noise arises from turbulence and, at high pressure ratios, from shock waves downstream of the restriction. It grows with mass flow and pressure ratio.

The sound level can be predicted before purchase: to IEC 60534-8-3 for gases and vapours, to IEC 60534-8-4 for liquids. The measures fall into two groups. At the source, low-noise trims that split the flow into many small jets and multi-stage pressure reduction are effective. Along the transmission path, heavier pipe walls, acoustic insulation and silencers are effective. Measures at the source also reduce the loading on the valve; measures along the path only reduce what arrives outside.