Thermal Oil Heater Piping system 2

[Rafleonard]

Hello everyone,

I'm currently designing a thermal oil system.
I have sized up the heater (16MBTU/hr) with 1,200 GPM flow rate.

I'm having issues calculating the pressure loss (due to friction) in my piping system. I'm confused when it comes to how to calculate this.
I know I have to add pressure losses due to friction in pipes, fittings, valves, heat exchangers, etc to get my total pressure loss.

I'm going to try my best to describe our piping system.

I have a 6" header coming from the heater, that pipe goes across our building (width), from that 6" header, I have (4) 3" sub-headers that are going to each production line, each 3" sub-header has (14) 1" service lines, at the end of each 1" line, there is a heating coil inside a tank.

Each 3" sub-header should have a flow of around 300 GPM (1200/4), then that 300 GPM is divided into (14) 1" lines, if all the tanks are heating, I should have an even flow rate (at least close) on each 1" pipe (300/14) of around 21 GPM.

should I only use the 1" lines (pipe + fittings + valves) to calculate my pressure loss?
or, since the 3" line has to fill up 1st. before going into the 1" lines, do I need to add the pressure loss on the 3" pipe as well?

I appreciate any help with this.

Regards.

 

[georgeverghese]

1. I've chosen DN150 here to keep dp minimal and keep more dp to be available to the heating coils even furthest down. To justify going down to DN100, given the much higher dp/length will require more detailed dp calcs to ensure T14 heating coil is not starved of pressure. You can verify if you want to save some money here. Given the extent of piping here, think there is minimal savings to be gained here, since most of the installed cost would go to labour and not to materials. Strictly speaking, even the return header on each of the subheaders 1-4 should be looped, but I'm assuming this wont be necessary given that DN150 line size should keep dp minimal even for long lengths to enable enough pressure at T14.
2. FIC- FCV is the better way to go. It is not affected by actual pressure upstream or downstream.
3. This enables supply to return header dp to be more or less the same for all headers.
4. Usual practice is to have the FT on the supply side. But you could just as well install it on the return header, provided it is well away from downstream of the teein from the FCV. There would be more turbulence and eddies at this downstream FT location given the tee in from the FCV, which increases the risk of readout error. We assume total flow into the system is the same as total return flow.

When you have time, you may do a detailed dp check that T14 on header 4 is not starved of pressure at full flow, at the lowest normal hot oil supply temp at high oil viscosity. Compare this with the dp available at T4 on header 1. If you do go for DN100 on these subheaders, you'll need to keep a sharp eye out for piping designers making late changes in the field to line routing and fittings.

 

[Patrick LaPointe]

This is an ideal application for a reverse return piping configuration. With such a piping arrangement, flow is inherently balanced and will divide evenly. No balancing valves required. I have used this piping configuration on thermal oil piping to heat log ponds at an OSB Mill in Grand Prairie, Alberta, Canada.

Pat

 

[Rafleonard]

georgeverghese:

The turndown ratio on the heater is 10:1.

If we are controlling flow in the return, and if one or more valves close, flow will go the the next system, making the return flow the same and the supply, that will increase the dp across each tank, will it not?

 

[georgeverghese]

That is a good turndown, there is most likely 2 staged burners.
Yes, dp across each tank will vary whenever one or more tanks reduces demand, hence each TCV's flow will swing accordingly, since you've got 56tanks. If you have sufficient volume in each tank, this should dampen out the effect of varying hot oil flow into the tank. If tank volume is low, a cascade control scheme would be necessary (master TIC resetting a slave flow controller).
Another way of addressing fluctuations in header pressure as tanks go on and off occasionally is to replace the suggested FIC-FCV with a dPIC-dPCV, which will maintain a constant dp between main supply and main return pressure, hence each TCV's flow will remain unaffected. You can set this dPIC set point such that it will also cover the min flow requirement of the 4.7MW hot oil heater and for the feed hot oil pumps. In reality, the FIC-FCV arrangement is, operations wise, the same as that for the dPIC-dPCV control loop for obvious reasons. But a dPIC-dPIC loop is simpler and easier to implement since an FE is not required.
On second thought, if the distance between the first DN150 tee off from the DN200 supply header to the last DN150 tee off is short, (and correspondingly so also from the 4 nos of DN150 tee ins to the main DN200 return header), then we wont need this looping arrangement on the return header. This is possible only because the dp/length for a DN200 main supply and return header is small.
Avoid taking 14nos of DN25 branches to the tanks on each DN150 header. DN25 weldolet and sockolet conns are not sturdy, prone to breakage and corrosion also. Instead, suggest say a DN80 forged branch tee off to a set of 3 tanks, so a total of 3 DN80 tees to the 10 tanks on the B side DN150 header. Minimise the length of flimsy DN25 piping runs. Avoid threaded piping connections in flammable high temp hot oil service.
Let me know if you need to see this dPIC loop and other amendments on a revised sketch.

 

[georgeverghese]

Would prefer to retain the FIC-FCV loop as I had shown in the sketch. Set point 1100gpm. By keeping this flow, dp across the DN200 main supply to DN200 return header will also be maintained, regardless of the no of tanks in operation. It has the added benefit of providing Operations positive confirmation of having exceeded the min flow requirement at the 4.7MW heater and for the pumps.
The TIC and TCV at each tank should operate on-off between control low and control high setpoints.
Presume you've got a low hot oil flow trip at the 4.7MW heater and also a min flow trip at each of the recirculating pumps.

 

[stewbaby]

You're at 8+ FT/S in that 1" pipe. I'd argue you want to be 1 to 2 FT/S at most...enough to have turbulent flow and push air out but with as much residence time as possible to allow heat exchange. If circuits are closed off, you'll push velocities even higher of course. I've typically put a 450 GPM pump on 12MM BTU hot oil systems and had more than enough flow (heating asphalt & emulsion tank farms). We use 2" Sch 80 finned coil tubes (sch 80 RAGAGEP for coils in asphalt and emulsion).

I use a program called PipeFlo to model such system pressure drop and flow rate / velocities.