Noise can be a problem in even low and medium pressure steam applications, resulting in issues like heavy vibrations or health, safety and environmental hazards. Avoiding excessive noise is a common requirement in many oil and gas plants, especially those related to gas processing, LNG, GTL or industrial gas production. The usual limit for noise is 85 dBA, but the limit for the whole project may be as low as 80 dBA.
Essential utilities
Steam is widely used in industries such as power production where high pressure drops and vibration are of concern. It is needed for various purposes in chemical and hydrocarbon processing: It can be used as a source of power, taking part in a reaction, like in steam reforming to produce hydrogen; or it can be used as stripping steam in columns and reactors, purging piping and equipment to keep them clean from fouling such as in delayed coking coke drum operations and steam cracking furnaces in ethylene production. Wherever steam is used, there is a likely need to control noise and large pressure drops. Special attention is required in the sizing and selection of valves to ensure that they are also clean, safe and reliable.
Compressors and surge
Compressor surge protection is a severe application requiring noise control. Capacity instability is accompanied by a characteristic noise known as pumping or surge. The resulting violent gas pressure oscillation can cause damage to the compressor in just a few seconds. The anti surge valve must be able to pass 100% capacity of a compressor, react quickly, handle high pressure drops and reduce noise while keeping a tight shutoff to avoid energy losses.
Answering noise demands
How does a valve manufacturer answer these extreme demands when noise needs to be controlled under various flow conditions and ever tightening noise level requirements? A good start is to look at the history and milestones of flow control noise abatement technology developments that are leading the way to modern technologies.
Rotary control valves have been used for decades in numerous processing industries. In the late 1970s, rotary control ball valves were already in wide use. The same design of the first low noise anti-cavitation, patented Neles Q-Ball introduced in 1979 still works on the same field proven principle of a multi stage trim of variable resistance depending on the valve opening. The difference between the noise level of a Q-trim ball and conventional ball can be up to 20 dBA. The Q-trim ball is also not sensitive to fluid impurities. The Q-trim ball became a common solution for many applications in steam, gas or liquid service. The next step towards higher pressure drops was taken in the mid 1980s when diffusers were introduced and sized so that their performance was optimised in conjunction with the control valve. The segmented ball control valve introduced in the 1980s added more possibilities to flow control. Using Q-trim in segmented ball valves gives a control valve with low recovery.
The late 1980s saw an extension in the applicability limits of the control butterfly valve when the silencer disc (Neles S-DiscTM) was launched. The solution was again very simple. The non-symmetrical pressure distribution pattern on both sides of the disc has been made symmetrical with a partial flow obstacle (S-Disc) inside the valve body. This design helps to eliminate the dynamic torque, and because of the more turbulent flow pattern, it lowers the high recovery behaviour.
Decades of experience and development form an essential basis to take further development steps in control valve noise abatement. Controlling noise and understanding the fluid behaviour in valves is based heavily on experimentation and supported by computational fluid dynamics.
The new Q2-trim is the second generation of Q-trim technology and is designed to reduce high aerodynamic noise to a tolerable level. The design itself follows the same principle as Q-trim and also utilises the same techniques of pressure staging, flow division, acoustic control and velocity control.
The idea behind the new development was to create high noise attenuation trim, which takes into account the 30 years’ history of the Q-trim and brings the noise attenuation to a new level of performance. The theory behind the design is based on the same physical phenomena that can be found with the Q-trim.
The science behind the development
What creates noise in the first place? The answer to this question is not straightforward. Although there are still unknown factors behind noise, theories have been built for them. The noise created by throttling gas or steam flow is called aerodynamic noise. There are various sources where the noise originates, for example, from the downstream turbulence of the valve, which can cause pressure fluctuations and pressure waves, high flow velocity and vibrations from shock waves. Once high noise has been generated inside a pipeline, it can propagate in several ways: inside the pipeline, along the pipe wall, along the pipe supports and into the surroundings.
Noise abatement can be done in several ways. The basic division of these is between ‘source’ treatment, like valve and trim modification where excessive noise is prevented, and ‘path’ treatment with dampening the noise generated by using silencers, insulation and heavier pipe schedule. Source treatment is the preferred choice, whenever feasible, because it also ensures reliable process operation by preventing the high mechanical vibration levels always associated with noise.
Source treatment of noise can be performed by at least four different methods: velocity control, acoustic control, location control and by using diffusers. The velocity inside a control valve trim can be controlled most effectively by using a multistage pressure drop and by increasing the valve trim outlet area such that the flow velocity and pressure at the valve outlet are at the minimum and the gas volume is at the maximum. The acoustic noise can be controlled in two ways: flow division into multiple streams and the modification of the acoustic field. Location control involves designing a valve trim in such a way that the location and the shape of the jet streams in, and especially leaving the trim, are such that the minimum amount of noise is produced. Dividing the pressure drop between a control valve and a downstream diffuser provides an effective way of further reducing the noise in cases where there is a constant, high pressure drop across the control valve and the flow is relatively constant. An attenuator plate can be used for noise attenuation in cases where the pressure drop is close to constant across the valve.
Velocity control
Controlling the maximum fluid velocity inside a control valve trim is a very effective way of controlling noise at subsonic flow velocities in the trim, as the acoustic intensity of a jet has been shown to be proportional to the sixth power of the flow velocity in a system with solid boundaries like a valve trim or a pipe.
Acoustic control
Acoustic control affects the noise level by means of acoustics. Two methods used in control valves are flow division into multiple streams and the modification of the acoustic field.
In theory, flow division into multiple streams is effective because the intensity of the noise generated by a single orifice decreases rapidly when the hole diameter is decreased. Thus, a number of small holes attenuates the noise more effectively than one big hole. A rule of thumb is that each doubling of the number of holes reduces noise by 3 dB.
Location control
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. The formation of turbulence in the mixing region between where the jet exits from an orifice and the gas flow at the outlet region as well as the attachment 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 a certain extent by intelligent valve trim design. One way to do this is to smooth the velocity profile of the jet by introducing a lower velocity gas stream alongside the jet.
As mentioned earlier, the further development of enhanced noise reduction trim is based on applying the known technologies in experimental research. This method has proved to be successful in providing enhanced noise attenuation for gas and steam applications. It provides very low pressure difference over the last stage, effective flow division to reduce noise level in low and high pressure differentials, avoiding resonances and extra turbulence as well as taking into account the insertion loss related to separate static resistors.
Sizing valves and predicting noise
Computerised control valve sizing and noise prediction arrived during the 1980s with sizing programmes, which acted as tools for evaluating the true performance of the control valve. Today, control valve sizing programmes are the key to appropriate valve noise prediction for overall noise control. Aerodynamic noise equations for control valve sizing have been defined by using international standards like IEC 60534-8-3.
Conclusion
The capability to avoid excessive noise is a common requirement in many oil and gas plants. Valves play a critical role in noise control. Wherever there are gaseous hydrocarbons processed or utilities such as steam, hydrogen, nitrogen and oxygen used in hydrocarbon processing, there is a likely need to control noise at large pressure drops. Special attention should be paid in applications, like compressor anti surge, to handle the extreme service demands.
There are various sources for aerodynamic noise generation. Several noise theories have been built to understand the complexity of noise generation and the factors behind it. The unknown factors make it more challenging to predict the noise behaviour in real conditions. Experience forms an essential basis for further development steps in control valve noise abatement. The new noise attenuation designs are based on a balance between the theory and practice.
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| Email: | steve.clark@valmet.com |
| www: | www.valmet.com/flowcontrol/ |
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