16 replies to this topic

[ulka_19]

Hello everyone,

 

I am a process engineer, working since two years in process design in an EPC firm.

 

Here I want to ask about the positioning of valves. Presently I am involved in a tank farm project. We are having heating arrangements for Bitumen storage tank via heating coils in tank and suction heater. For this heating we have a hot oil system so my question is that if given the choice between gate valve and globe valve which valve should be positioned on hot oil supply and return line. My drawing says which I have not prepared that globe valve on return line and gate valve on supply line.

 

So I want to know how we decide on these factors.

 

Any literature about the recommended practice of valves or practical uses of valves would be appreciated.

[fallah]

ulka,

 

The globe valve is for oil flow regulation and the gate valve for coil/oil isolation, then it would be better the gate valve to be in supply line and the globe valve in return line...

[ulka_19]

Thanks for your reply but that is what I am exactly asking why are we controlling the return line.

 

This control could be achieved even if the globe valve is on supply line.

[fallah]

ulka,

 

Ok, they can be vice versa i.e. the globe in supply line and the gate on return line; no considerable difference in heat transfer rate but if there would be a need to coil maintenance and subsequent coil isolation, because of making better isolation than the globe it's better the gate to be in the supply line...

[ulka_19]

Ok, Thanks

[aroon]

I would say, the configuration, gate valve at the inlet and globe valve at the outlet will have better performance than the vice-versa. The reason behind that is, you will have hot oil at higher pressure i.e. at inlet header pressure, which will allow hot oil to be slightly more intact with coil surface. Due to this you will have some benefit in heat transfer because of reduction in boundary layer thickness on inside surface of coil. In addition, slight increase in density of hot oil will also give certain benefit to increase Raynold's number, which thereby beneficial in heat transfer.

 

Hope this may be helpful!!

 

Regards,

Aroon

[ulka_19]

Hello Aroon,

 

I take your first point but the second part that when the density increases Reynold Number increases, I dont agree because as the density increases viscosity is also changing and in my case following situation arises:

 

Inlet Temp    230^oC Density- 806.6 kg /m3, Viscosity-0.26 cP , Density/ Viscosity- 3102

Outlet Temp 210^oC Density- 821.7 kg /m3, Viscosity-0.29 cP  , Density / Viscosity- 2833.4

 

Thus Reynold No. should decrease.

 

Please correct me if I am wrong.

 

Thanks.

[aroon]

Why you are considering viscosity at 210⁰C for comparison in other case? It should be same value 0.26 cP at 230⁰C.

[fallah]

ulka,

 

as i mentioned in post mo. 4 the heat transfer rate wouldn't be so different in two mentioned valves arrangements, because the physical properties of the oil wouldn't be so changed due to pressure change based on which the heat transfer rate can be considerably affected....

[rabindra]

I do not think it would make any difference. The Cooling Water Supply and Return lines also have a gate valve in supply line and globe in return line. If you find any standards let us know.

[ulka_19]

@ Aroon:

 

Density change is accompanied by viscosity change. Density & viscosity both properties will change with temperature and pressure so why are you considering only density change?

[ulka_19]

I too think that heat transfer wise there will be no significant difference and as Fallah mentioned this arrangement is for the ease of maintenance. Anyways any literature related to thic article would be helpful if anyone has then please share.

[rahulavyas]

IS Your Hot Oil is Dowtherm - A?

Generally control valves are installed in outlet for Hot oil service by many Detail engineer contractors, this provides ease of operations and heat transfer also. Secondly If you are controlling at inlet samll amount of flashing may also occur which impacts the heat transfer a bit.

[latexman]

A phone call to your heat transfer fluid technical support may be a worthwhile discussion.  I have had many discussions with our heat transfer fluid technical support over the years and he has always provided good advice along with good documentation (case studies, recommendations, oil analyses, etc.).

[PingPong]

Position of globe valve has no influence on the heat transfer, as fallah already explained.

 

In cooling water systems it is custumary to put the globe valve in the outlet line. This creates the highest cw pressure inside the exchanger and thus the lowest chance of water vaporization on the hot tube wall, causing salt deposition, in case the cw is used to cool streams hotter than 100 ^oC.

Therefor this valve positioning became standard practice.

 

However: one should always use ones own brain before blindlessly apply a standard. If for example there is no risk of cw vaporization, but cw leakage into the process fluid is much more problematic than leakage of process fluid into the cw, it might be better to have the globe valve upstream the exchanger, if that avoids a cw pressure above the product pressure. Also, if there is no risk of cw vaporization and the process fluid is a high pressure gas, I prefer to have the globe valve upstream the exchanger to minimise the pressure peak inside the exchanger in case of a tube burst.

 

The same philosophy should be applied to choice of globe valve location in any other heat transfer systems. Always ask yourself: what is the least problem in case of tube leakage: heat transfer fluid leaking into the product, or product leaking into the heat transfer fluid?

Edited by PingPong, 29 March 2014 - 07:55 AM.

[aroon]

@ Aroon:

 

Density change is accompanied by viscosity change. Density & viscosity both properties will change with temperature and pressure so why are you considering only density change?

Ulka_19,

 

Viscosity will not have that much impact due to increase in pressure as it does for density. But temperature will have substantial impact on both density as well as viscosity.

 

Now here our concern is about pressure so you may please check Raynault's number for different pressure @ same temperature. You will find increased number when pressure increases. You may also check effect of boundary layer on heat transfer, which will also help in clarifying your concern.

 

Here I am not saying that any one of the arrangement wrong or something. However, just wanted to highlight some benefits of the one arrangement over another. As an engineer if you visualize/co-relate pressure and temperature parameter with system then increase in these parameter will bound to have following impacts on the system:

1. Increase in internal energy of the system as here you are adding energy into the system in terms of pressure.

2. Increase in internal energy will increase randomness of the molecule, which will thereby increase the turbulence and reduce the boundary layer.

3. Increase internal energy which itself means enhance capacity to do necessary work (here it is heat transfer). 

 

Hope it may be helpful to understand the effect on heat transfer.

 

- Aroon

[aroon]

I too think that heat transfer wise there will be no significant difference and as Fallah mentioned this arrangement is for the ease of maintenance. Anyways any literature related to thic article would be helpful if anyone has then please share.

 

I agree that there is no big gain. However, in today's engineering world, where energy conservation has gained prime importance, if changing something without cost is going to add even small benefit then everyone should adopt it.

 

I believe there are no literature available on this particular arrangement. But literature on effect of pressure on heat transfer/molecular movement could be useful to clarify your doubt.