Leaders in the process industries realize that good process control performance is an essential element in achieving world-class reliability as well as optimizing overall process efficiency. Since control valves are the only devices in the process loop that actually “move” to adjust the process,their performance is critical. The best way to achieve excellent performance
is to initially select the most appropriate final control valve for the application and then to maintain its performance over time.
Noise :The maximum noise level emission from a control valve manifold installation,including contributions from piping elbows and reducers, shall not exceed the following limits for any specified operating condition.
- 85 dBA for process control/feed regulation, continuous letdown,intermittent letdown daily service and recirculation daily service applications.
- 90 dBA for infrequent letdown and recirculation applications.
Noise shall be calculated in accordance with the methodology outlined in ISA SP75.17 and IEC 534-8-3 guidelines. The maximum noise levels are specified in terms of equivalent continuous A-weighted Sound Pressure Levels (SPL) with an upper tolerance of +0 dBA. Seller shall be required to include inaccuracies of their quoted noise levels and shall guarantee that the noise emission from control valves, including upper tolerance, shall not exceed the above stated limits for any specified operating condition.
Control valve noise shall be treated at the source except as noted below. The provision of low-noise multipath multi-stage trim designs are generally most cost effective. Diffusers, baffle plates and silencers, either inside the valve body or downstream of the valve, shall not be permitted for the following applications:
– Process control/feed regulation
– Continuous letdown
– Intermittent letdown in daily service
– Recirculation in daily service
For other applications, special attention shall be given to the limited flow rangeability of these devices (i.e., noise reduction from these devices is generally reduced at low flow conditions). Noise generated by diffusers or orifice plates shall be calculated by ISA SP75.17 or IEC 534-8-3 as a separate system from the valve trim and included in the overall noise source.
A maximum of 5 dBA credit may be used for the application of insulation for noise reduction. Seller shall submit detailed noise calculations with the proposal in accordance with the methodology outlined in ISA SP75.17 and IEC 534-8-3 guidelines. The calculation basis and results shall be shown for manual or computer calculations. Seller shall guarantee control valves supplied meet the above requirements.
Vibration and Erosion Limits
Proper control valve selection shall ensure that the required energy can be dissipated without exceeding the maximum vibration levels in the piping system and without exceeding the wear properties of the trim material. Seller shall select control valve trim design that facilitates maximum reduction of control valve induced vibration and trim wear.
Control valve vibration and trim erosion can be reduced by multi-stage multipath trim designs. In order to minimize vibration, Seller shall select control valves based on Table 12.3 of the ISA Guide, “Control Valves, Practical Guides for Measurement and Control” summarized below:
1. Liquid Service Applications
The design shall provide a sufficient number of discrete pressure drop stages to maintain the trim exit velocity below 30 m/Sec (100 ft/Sec) for single-phase liquids and 23 m/Sec (75 ft/Sec) for cavitating, flushing, and or erosive services. Seller shall specify the number of discreet pressure drop stages required and provide body and trim exit velocity calculations with
Proposal. The calculation basis and results shall be shown for manual or computer calculations.
2.Gas Service Applications
The design shall provide a sufficient number of discrete pressure drop stages to maintain the trim exit fluid velocity head (Vh) below 480 kPa (70 psi) for continuous or intermittent duty and 1030 kPa (150 psi) for infrequent duty. Valve trim exit fluid velocity head shall be calculated based on ISA Guide, “Control Valves, Practical Guides for Measurement and Control”.Seller shall specify the number of discreet pressure drop stages required and provide body and trim exit fluid velocity head calculations with the proposal. The calculation basis and results shall be shown for manual or computer calculations.
3. Cavitation, Choked Flow and Flashing
ISA S75.01, the ISA Guide “Control Valves, Practical Guides for Measurement and Control”, and the Seller’s valve cavitation index data shall be used for determining the severity of cavitation, choked flow, or flashing conditions in the control valves. Seller shall provide a control valve that eliminates cavitation damage.
The design of valves in cavitation and flashing service shall be based on Chapter 7 of the ISA Guide, “Control Valves, Practical Guides for Measurement and Control” and include the following techniques for Cavitation-Resistant Valves:
– Reduce the pressure in multiple stages
– Direct flow away from the valve body and pipe walls
– Break the flow into many small streams
– Force the flow through multiple turns or tortuous paths
The design shall provide a sufficient number of discrete pressure drop stages to maintain the trim exit velocity below 23 m/sec (75 ft/sec) for cavitating and flashing service. Seller shall specify the number of discreet pressure drop stages required and provide body and trim exit velocity calculations with Seller’s proposal. For multi-constituent fluids, special
Consideration shall be given to the potential for cavitation of the minor components in the fluid. Contingencies on the minimum required control valve cavitation index shall be applied to compensate for inaccuracies in process data and
inaccuracies in Seller’s control valve cavitation index data. Buyer shall carefully evaluate Seller’s selection of a control valve that eliminates cavitation.
Seller shall select the control valve flow characteristic to meet the rangeability requirements as defined in Section 5.1.4, Minimum Rangeability, and to provide stable control over the required range of operating conditions. The seller should select the control valve flow characteristic specified in the control valve data sheets.