In controlling the fluid, to reach the maximum efficiency and smooth control we have to select the suitable internal part (wetted part) of a control valve. With the valve trim the flow characterization, the desired leakage class and smooth controlling can be achieved.
Control Valve trim is the internal components of a valve that modulate the flow of the controlled fluid. In a globe valve body, trim would typically include closure member, seat ring, cage, stem, and
stem pin. The body, bonnet, bottom flange, guide means and gaskets are not considered as part of the trim.
The trim can be specified as per below:
1. Trim classification as per seat material
Seat material selection for trim will affect to the desired leakage class for the valve.
As per ISA S75.05:
“Soft seated trim: Globe valve trim with an elastomeric, plastic or other readily deformable material used either in the valve plug or seat ring to provide tight shutoff with minimal actuator forces. See ANSI B16.104 for leakage classifications”
Soft seated material is used for tight shutoff, low temperature and not service a flashing, cavitating, or other slurry services.
This soft seated usually damage by any debris which flowing with the fluid. It can be happen in the first stage of plant commissioning which the pipe still including by some debris of construction welding material.
Metal seated trim usually used for severe service high temperature, flashing, cavitating, and slurry services with maximum leakage class V. The seat material such as 316 SS, Monel, etc. Best used for the severe service, flashing, cavitating, and slurry services.
Mostly the leakage class IV for control valve is good enough for process control except required by any specific process. To achieve the tight shutoff for emergency shutdown then the quarter turn off shutdown valve with tight shutoff will installed in series with the control valve.
Leakage Class as per ANSI FCI 70-2 (superseding ANSI B16.104)
2. Trim as per guided type
Post guided valve equipped with guide bushing or bushings fitted to posts or extensions larger than the valve stem and aligned with the seat. This post guided valve usually used in the general service fluid that doesn’t exhibit cavitation, flashing, or high noise behavior.
Cage guided valve which the valve plug fitted to the inside diameter of the cage to align the plug with the seat. This cage guided valve usually used in the fluid service that have cavitation, high noise or flashing effect.
3. Control Valve trim for special services
For special severe services, the trim control valve can be selected to minimize the impact of severe condition.
Anti cavitation trim: anti cavitation trim can minimizes cavitation damage to valve components by split the pressure drop in each perforated cage and the lowest pressure drop will occur without exceed the vapor pressure. The other type of anti cavitation trim is by controlling the location and concentration of cavitation vapor bubble implosion in an area away from metal parts. The flashing will guided to the center of the retainer and flashing occur in the center that minimizing the impact to the plug and valve body.
Anti-noise trim: A combination of plug and seat ring or plug and cage that by its geometry reduces the noise generated by fluid flowing through the valve.
Anti noise trim intended to reduce the aerodynamics noise when the turbulent energy released as the pressure of the fluid drops at the trim. The noise reduction can perform by reduce the turbulent occurs in the trim by using vertical flow cage trim. This method can reduce noise up to 18dB
The others method is by dividing the flow by the perforated trim, The reduction is achieved by the fact that the smaller openings shift the noise to a higher frequency. The higher frequencies are attenuated more as the sound passes through the pipe wall than are the lower frequencies. Also, because the human ear attenuate higher frequencies, both the measured and perceived noise is decreased. This method can reduce the noise up to 30dB
Pressure balance trim.
Some high pressure drop applications or valves with large seat diameters may require pressure-balanced trim. This method is an economical choice to avoid over sizing the actuator. The arrangement of cage and valve plug is to equalizing the pressure from lower side or valve by provide a small transfer hole in the valve plug to allow the pressurized fluid fill the upside of the valve plug and reduce the force requirement of the valve actuator when dung the valve close action. One of disadvantage of this arrangement, it’s have low leakage shutoff capability.
4. Valve Trim by characterization
Characterization of valve can be perform by design the trim. There are 3 types of flow characterization in control valve:
– Quick Opening
– Linear and
– Equal percentage
The trim can be modified to perform the flow characterization
Withe the correctly choosen the trim will ensure the proces control of fluid will optimal and will affect to great efficiency of Process. Any question let me know.. sent an email to munzir_888@yahoo.com.
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