Control valves and velocity 1

[fred181]

When evaluating a control valve for high velocity, at what cross section are you calculating the velocity? I have always calculated at the valve exit.

This is a steam pressure reducing application. I am evaluating a selection made by a manufacturer's rep. The rep uses a term "body velocity" and uses a larger cross sectional area for his calculations provided with his valve specification sheet (19.02 in2 vs. the 12.57 in2 I would use at the valve exit for a 4" valve).

4" full port control valve
steam flow: 20,000 lb/hr
inlet pressure: 270 psig (saturated)
outlet pressure: 10 psig
specific volume at outlet: 18.72 ft3/lb (superheated)

With these conditions, I get a valve velocity of 1,191 ft/s or 0.7 Mach at the valve exit, which is excessively high. The manufacturer provides 787 ft/s or 0.37 Mach.

Anyone have some insight as to which approach is more valid? I also understand general guidance to be that velocity should be limited to somewhere in the range of 0.3 to 0.5 Mach.

 

[davefitz]

The max velocity will be a function of both the minimum open area and the valve's internal geometry. In the most extreme case , if there is a sharp turn inside the valve, then the min effective flow area might be as low as only 68% of the apparent open area due to the vena contracta effect or due to oblique shock waves. These effects can be made to appear to be less of a constraint if the mfr provides an outlet perforated baffle plate, and then might be justified in claiming a lower exit velocity.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[hacksaw]

Your discharge piping is undersized. The discharge velocity, needs to be less than 100 ft/s in most applications.

 

[Snickster]

What is the temperature and compressibility factor you are using at the outlet? This looks to be a ball control valve such as a V-ball by Fisher correct? So the inlet steam is saturated at 270 psig or about 412.5 deg. F? What is ID 4" pipe? What value are you using for steam ratio of specific heats?

 

[shvet]

Anyone have some insight as to which approach is more valid? I also understand general guidance to be that velocity should be limited to somewhere in the range of 0.3 to 0.5 Mach.

Threshold erosion velocity may be considered:
- for stainless steel 80-100 m/s
- for carbon steel 30-80 m/s
Relevant information can be found here.

https://www.eng-tips.com/viewthread.cfm?qid=428229

 

[TugboatEng]

If your steam quality is low you may want to reduce those numbers substantially.

 

[KieBry]

I think the Burkert FLOWave measures the velocity for you.

 

[casflo]

The steam passing through the valve has an expansion close to the isenthalpic. With an inlet pressure of 270 psi and the valve discharging to a zone at 10 psi, the valve has choking conditions in its throat and the pressure here will be approximately 135 psi. Therefore, to calculate the velocity of the steam at the valve throat, you must cosider the 135 psi value to obtain the specific volume and not the 10 psi value.

 

[Snickster]

I disagree with cashflo. The flow is sonic as he states at the downstream end of the ball control valve. Flow is regulated by reducing the area of flow by closing the ball valve not by frictional throttling because sonic flow is attained due to the very low downstream pressure, therefore flow is not isenthalpic flow but closer to isentropic expansion with some friction. Any degree of opening of the valve with the downstream pressure set at 10 psig will always produce sonic velocity in the downstream passage of the partially closed ball vavle. Therefore flow is determined by sonic flow condition pressure, temperature and density at the critical pressure and flow is reduced soley by reducing the flow area of the valve not by frictional throttling. To get from upstream 270 psig to 10 psig in the downstrem passage of the valve there is expansion of the gas where the flow work energy and internal energy (i.e., Enthalpy) is converted to velocity therefore the temperature must drop but at the same time as the flow travels across the valve there is some friction loss too. If flow was ideally isentropic expansion to sonic velocity at the downstream passage then temperature at downstream passage of valve where sonic conditions exist would be To(2/k+1) where To is the upstream temperature. With friction the temperature would be somewhat less I believe. Therefore to calculate the downstream flow conditions at 10 psig in the downstream 4" use the lower temperature not the upstream temperture before the valve considering from sonic conditions at the dwonstream passage to the 4" pipe flow it is an isenthalpic expansion due to sonic shock waves reducing the pressure from the critical flow in the valve passage to 10 psig where temperature remains constant. This will result in a higher density fluid and lower velocity than if you use To of the upstream saturated steam of approx. 412F. In any case it seems the 4" downstream pipe is too small and velocity is normally limited to 60 ft/sec for noise and for erosion-corrosion velocity is limited to API recommendations which I believe is about 150 ft/sec (check). As far as the valve goes I would check with manufacture whether there is any issues with regulating the flow in such conditions where sonic velocity is acheive in passage at somewhere between 100 psig and 200 psig critical flow pressure then the pressure is droped suddenly to 10 psig. Can the valve handle this?

 

[LittleInch]

Snickster.

Can you please break up your replies into paragraphs to make it more readable, especially on mobile devices. It's a big block of text which is difficult to follow.

Also this is steam not gas and is a control valve (type not specified), not a ball valve.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[pierreick]

Hi,
Good to read:

https://www.linkedin.com/pulse/control-valve-sizing-what-choked-flow-j-brooks-honeycutt-iii

Pierre

 

[LittleInch]

I'll confess I don't really follow all of this, but there is a detailed explanation here

https://www.spiraxsarco.com/learn-a...ol-valve-sizing-for-steam-systems#article-top

What exact sort of valve is this? A single full port or double port or ??

Clearly the saleman is looking at some other cross section, but without knowing what the valve looks like it's not possible to say.

Can you post the valve spec sheet where this is listed?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[casflo]

Snickster,
A throttled valve has a significative frictional pressure drop, therefore the expanansion is closer to isenthalpic than isentropic.
For example, in a nozzle, as in the case of a steam safety valve, the friction is very low and the expansion is close to isentropic.
In any case, the difference of the expansion coefficients for the saturated steam of both expansion types is minimal.

 

[Snickster]

Even if the flow was completely frictional the flow would not be isenthalpic since the velocity increase at critical flow sonic condition at the valve exit orifice would require that the enthalpy decrease to provide the necessary energy to increase the kinetic energy of velocity. There is really no telling what ratio the pressure decreases across the valve due to frictional losses versus pressure loss due to ideal gas expansion of course in a nozzle it would be moslty isentropic expansion with some friction. The manufacturer should have calculations for their valve that gives this information. I assume that for such a short distance through a valve that the frictional pressure drop is at least equally shared by the pressure drop due to expansion, and I would think but not sure that the frictional pressure drop is the smaller.

Steam is a gas although a special gas. The original post indicated that it was a full port valve so it appears to be a V-ball type control valve like fisher makes but not sure.