Control Valves - Cage vs Labyrinth

[Valving]

Regarding control valves, I dont like having blind confidence in vendors.
There are a few companies having labyrinth design and I was told that when
the reducing steps exceed 7 stages with cages, labyrinth design becomes feasible.
Of course this step requirement is limited with ISA figures (30 m/s - 480 kPa : trim exit velocity)
On the other hand, I was also told that labyrinth design is not good for HP steam applications.
For example, by turbine bypass, labyrinth design is very much expensive as far as I see
on the other hand, regular cage design are told not to be very reliable or everlasting.
It is not easy to learn which is correct by experience unless you operate power plant for 10-20 years, I believe.
so I want to share this issue in the forum.
Additionally is ISA a guideline or a general industry handbook?

Could the experienced ones share their ideas, I would appreciate.

 

[gerhardl]

As you have got no answers yet, I would like to try to comment:

Even if I have been on the vendor side myself, I agree that you should not trust a vendor 'blindly'. On the other hand, good vendors are updated on the latest technology of the brands they represent, and would not like to loose reputation on giving you false or incorrect advice. In addition a good vendor could also advice you on placement or additional equipment to improve performance of the valve, and help you to correctly size the valve.

What I would do is to give all data for an actual HP application in your plant, where you know you will have to do a replacement or renewal. Important figures: flow charecteristics/amount (max, min and 'normal'), flow duration, and intervals between flow situations, also piping arrangement with steamtraps and filters etc. if any.

Ask for a preliminary offer for a valve, give out that selection will be based on an evaluation of (best estimate of) cost over lifetime, asking for figures for standtime between service, typical service parts and cost or service, best technical solution, and alternatively reduction in price by selecting 'next best but adeqate' technical solutions/materials. Ask for technical comments.

A very large part of early wear of HP valves stems from wrongly sized, selected, false mounted (check orientation, straight stretches, steamtrap, filters, bypass, isolation valves etc. etc.) Cost cutting when purchasing (too low quality, cutting out additional piping components.

Answer to your question: Seen on this background the one valvetype might as well come out on the top as the other, providing 'everything else' is correctly stated, selected and mounted.

 

[Valving]

thanks Gerhardl, for your time and comments. I believe this subject is a blind area as I could never locate a comparison or further study but I am willing to dig deeper.

 

[casflo]

The main reason to decide between a labyrinth design or a cage design valve in liquid flw, is the cavitation. The labyrinth design usually has cavitation coefficients lower than the cage design, but they must be provided by the valve suppliers.
For other services as liquids that may flash, steam or gases you must take into account the valve materials and the internal valve velocities in order to diminish the erosion damage and the noise.

 

[Valving]

Casflo thanks for input.
Do you have any specific and clear statements for these coefficients?
Or any literature that you could advice.
I never saw a manufacturer indicating these however of course it could be requested.
One further thing : for flashing media, beside erosion issue, you dont indicate any difference?
Do you think that labyrinth design worth to pay 1.5 or double price?

 

[casflo]

The minimum cavitation coefficients of the labyrinth and multistage cage designs are close to 1. Note that these coefficients depend on the valve opening and 1 is the minimum theoretical value that phisically it´s possoble to reach.
As an example, look for the Flowserve Cavitation Control brochure.
For flashing media take into account that you must design all together the valve and the downstream pipe. In my experience based in the design and operation of nuclear and thermal power plants, the cage valves with the appropiated materials and the downstream pipes of alloy or stainless steel, is a good design.