Can the old valve do the new job?
When looking to increase the productivity of an existing plant, engineers have to take control valves into account. This second case study for an existing application to control a liquid medium flow presents the troubleshooting measures to increase a plant's productivity to meet current market demands.
This example looks at an existing 8" cage valve optimised for 85 dB (A) with an additional multihole baffle and provides an easy method to obtain new valve parameters without having to start a new time-consuming total plant pressure loss calculation. At least with just one more powerful pump the new operating point can be kept controllable. However, severe cavitation increases the noise to 96 dB (A) with the existing control valve. The unique new development of an anti-cavitation trim 'AC Trim System' can solve cavitation problems and reduce the sound < 85 dB (A) within the entire control range (see the paragraph headed 'Control valve failure and troubleshooting').
The software provides the user with new calculation methods including graphic supports to help check the control valve performance as well as leading to the most economic solution to reduce noise.
Can the existing control valve be updated taking noise limitation of 85 dB(A) into account?
The case study is based on a real situation where the productivity had to be increased, while keeping the noise level (SPL) within the existing regulations. If the old DN 8" valve just fulfilled the noise requirements of 85 dB(A) by using a baffle or silencer, then the solution for the revised valve presents a real challenge.
The cage retained seat valve has been operating for some five years without complaints, but cannot be used after debottlenecking to control 30% more flow because of increasing sound-pressure-level >95 dB(A). The new pump-impeller increases the power to such a level that there is no economic solution available with the old valve.
Fortunately, Samson's research and development program has resulted in a unique anti-cavitation 'AC Trim System'. This fulfils the 85 dB(A) requirement and replaces the existing cage trim design. Samson's AC Trim I System features a CFD optimised, seat-guided parabolic plug top.
Successful debottlenecking after increasing the pump power
Sound optimisation for <85 dB(A) noise limit is made a reality using the AC Trim System, believed to offer the highest xFz characteristic for a control valve. The new pump impeller increases the plant upstream pressure, and of course the power and noise as well. The old cage valve would generate 96 dB(A) under these conditions. There is no way of keeping to the low noise level with the existing valve. The new valve with AC Trim System shows no cavitation at the operating point 380 t/h 65 dB(A) and less cavitation <85 dB(A) in the entire range of control.
Predictable control valve sizing difficulties with sub-critical flow conditions
It is widely understood that sensitive valve sizing areas exist with supercritical gases and slightly sub-cooled or non-sub-cooled liquids (flashing). Vapours and gases are calculated with the isentropic exponent k as one of the property values. Some hydrocarbons, eg, ethylene, are near or above the 'critical points t_ crit. and p_crit.' during the process.
The sizing standard IEC 60534 2-1 includes an information table with typical isentropic exponents used for steam and gas sizing. The total range 1 < k < 2 is well-known for all compressible fluids.
However, it is less-well-known that values 2 < k < 20 exist with supercritical fluids near and above the property critical point.
Samson broached this subject with the help of a latest development in precise property calculation, published at the Ruhr University of Bochum for more than 60 industrial gases - and this has been integrated into its CONVAL software.
The third case study showed tremendous sizing differences in the flow calculation for an ethylene application at the critical point of properties by using the real isentropic exponent much larger than 2. This can have a negative influence on plant safety valves and other devices. In the past, devices for supercritical flows were oversized because the wrong isentropic exponents and 'choked flow limits' were used. Samson is interested in initiating open discussion on how to define and handle this phenomenon and on how to validate it with measurements.
Poor planning and going cheap bring predictable problems
The CONVAL software features indicators (red shading on the graphs) to warn of the onset of cavitation and flashing at smaller loads. As long as control is above Op2 there is no risk, but with the control of smaller loads (<Op2), the valve DN is too small. CONVAL calculates real thermodynamic flashing conditions with about 60 hydrocarbons (see Table 1) and recommends the minimum valve DN to avoid critical outlet velocities.
The calculations resemble steam table mathematics. This is based on a reliable source, the 'Lehrstuhl für Thermodynamik Fakultät für Maschinenbau der Ruhr-Universität Bochum' (www.ruhr-uni-bochum.de/themo/index-eng.htm)
Control valve failure and troubleshooting
(Ranging from seat guided V-port to CFD optimised trims and their applications.)
There are different solutions to avoid critical sound and mechanical valve failure. Here we look at anti-cavitation valve trim designs and noise attenuation devices and discuss their advantages and disadvantages as well as their application limits.
Note that having velocities that are too high at the valve moving parts and at valve outlet is the biggest cause of valve failure - especially where corrosive fluids are handled. Samson specifies firm limitations on its valve outlet velocities, and other parameters, for the high performance V-port trim for general service; flow dividers I and III and downstream low noise devices for gas and steam pressure letdown.
If the V-port trim sound pressure level (SPL) is not acceptable for liquid applications or cavitation and corrosion must be avoided in general, the unique AC Trim System is recommended with top and seat guided plug; it is vibration-free and dirt-insensitive.
The max. pressure differential 2,5 to 4 MPa depends on the fluid properties. For case histories of troubleshooting with the AC Trim system see Figure 1 (a) and (b). Further, Table 2 gives an overview of the advantages and disadvantages of different trim designs.
AC Trim III system multistage design the AC Trim III System is suitable for use in liquid applications to avoid cavitation, wear and noise (see Figure 2). Features include top and seat guided plug, allowing vibration free, dirt-insensitive performance, with/without pressure balance, pressure differential 2,5 up to 12 MPa; AC Trim V System- 5 stages-12 MPa < .p <20 MPa. Three and five stages in the cv range from Cv=1 (3 stages) to Cv=116 from DN 1 to DN 6 inch in globe and angle type valves are used in case of severe cavitation problems eg, high .p together with a larger control range qmin to qmax. Typical applications are feed-water start-up valves, refinery valves, snow gun valves, injection valves, boiler applications, high pressure letdown service, etc.
A hidden valve enemy
Critical outlet velocities need to take priority, overriding 'quick and dirty' sizing philosophies, if selecting too small a valve DN taking only the calculated Cv value into account. High flow capacity valves (Cv/DN2) need to be selected with care when critical operation conditions are involved. Rule of thumb to avoid mix phase flow: in case of pv equal or near to p1 avoid any pipe restriction for 20 x DN valve upstream, this means no elbows, no manual valves, no pipe reducers.
Sensitive sizing areas special valve DN selection by giving priority to the outlet velocity condition of cavitation and flashing in liquid application and gas and steam pressure letdown, taking important piping parameters into account.
In case of flashing conditions, the average outlet velocity has to be calculated for the mixture of liquid and wet steam or vapour. Severe pipe vibration and valve damage can be avoided if the valve outlet diameter restricts the outlet velocity to less than 60 m/s (average of 0,7 Ma of mixture sonic speed). Samson has developed equations of state for flashing outlet velocities used in CONVAL for all fluids in Table 1.
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