Valves promise a quiet revolution

London – ?Noise abatement is moving up the regulatory agenda, with companies in the UK, for instance, now required to protect workers from exposure to 80-87dB of noise - as averaged over a working day or week - and a maximum noise of 135-140 dB in a working day.

With the emphasis also on reducing noise at source, there is growing interest in abatement technologies and designs that can help process operators reduce noise emissions from plant and equipment.

The range of options available to equipment designers in this area is highlighted by a recent development at Metso, which is already being used in the power, oil & gas, chemicals and industrial gas industries.

The Finnish group’s new Neles Q2-Trim gas control valve combines several techniques, including velocity control, pressure staging, flow division, and peak frequency shifting, to reduce noise by up to 30dB.

Explaining the concept, Metso said controlling the maximum fluid velocity inside a control valve trim is a very effective way of controlling noise at sub-sonic flow velocities in the trim. This is because the acoustic intensity of a jet is proportional to the sixth power of the flow velocity in a system with solid boundaries like a valve trim or a pipe.

Meanwhile, when the pressure drop across a control valve is taken in multiple stages, the total pressure drop is divided into several smaller successive pressure drops. The successive stages are spaced in such a way that the gas pressure is allowed to recover to an intermediate pressure and velocity before the next throttling stage.

This intermediate recovery of pressure and the resulting velocity reduction, said Metso, prevents the fluid from reaching the velocity it would have in a single-stage pressure drop system.
“In other words, the smaller the pressure drops, the smaller the recovery and fluid velocity increase downstream of the orifice,” it said.

Flow division into multiple streams is also effective because the intensity of noise generated by a single orifice decreases rapidly when hole diameter is decreased.

Thus a number of small holes attenuate noise more effectively than one big hole. A rule of thumb is that each doubling of the number of holes reduces noise by 3dB.

Location control involves designing a valve trim in such a way that the location and the shape of the jet streams in the valve trim, and especially leaving the valve trim, are such that the minimum noise is produced.

Also, turbulence and interaction of shock waves - generated during throttling if the flow reaches sonic velocity in the valve - are major sources of noise that can be controlled to an extent by intelligent valve trim design, added Metso.