High Flow Velocity Through Pipe -BFW

[robertj9]

Hi Everyone,
I have a Boiler Feed Water (BFW) recirculating line, that is being redesign. I have some models on it, and it looks like the velocity in that line (1-1/2" cs sch 80) is about 18ft/s, then it has a reducer to the inlet of DA (1" cs sch 80) In which the speed is about 44ft/s.

Is this velocity too high that it will cause erosion? Must of the people that I talked said that 10ft/s is what they recommend, but I will like to have something written that I can use as reference I order to demonstrate this flow will cause erosion.

 

[EmmanuelTop]

That's way too much for BFW (and any water) system. Velocity should be between 2 and 16 ft/sec, depending on the line diameter. See thread

http://www.eng-tips.com/viewthread.cfm?qid=391381

and Table 2 at

http://petroleumcorner.com/line-sizing-criteria-philosophy/

Dejan IVANOVIC
Process Engineer, MSChE

 

[bimr]

It would be difficult to be able to get the water to flow at 44 ft/sec. You would have a significant head loss as well as erosion.

A good reference is Cranes. You can purchase the Cranes Technical Paper 410 reference:

https://store.flowoffluids.com/shop/publications/TP410.html

Reasonable pipe velocities depend on the application. There is no correct velocity for all applications. Here is a general guideline taken from Cranes.

Reasonable Velocities for the Flow of Water through Pipe:

Boiler Feed.............8 to 15 ft/sec
Pump Suction ............4 to 7 ft/sec
General Service.........4 to 10 ft/sec
City.......................to 7 ft/sec
Transmission Pipelines...3 to 5 ft/sec

Go to a basic hydraulics book. Try Cranes Technical Paper 410 as a reference for the above velocities.

http://www.eng-tips.com/viewthread.cfm?qid=111206

 

[MJCronin]

I believe that it is important to note that the operation of this BFW recirc system is not continuous.

In most power plants, the bypass should only operate rarely and only to protect the BFW pump. Is that the way that the system DOES operate ???

Because of the rare operation, I am not that excited about erosion/corrosion. The 18 ft/sec velocity versus the 15 ft/sec is not that significant, IMHO.

However, the DA inlet nozzle seems to be truly undersized and will probably erode. Was the DA reused from another plant ????

(The typical BFP recirc line is subject to large pressure drops and severe vibration and has failed in the past)

What is the line temperature and pressure ? What kind of device is used in the line to drop the pressure ????

MJCronin
Sr. Process Engineer

 

[robertj9]

Hi Guys, Thanks for your reply!

This is a new boiler, that we purchased. We damaged the pumps before, due to not meeting the minimum continuous flow through the pumps. It looks like the company under design this re-circulation line. I'm trying to demonstrate to them that we need to change the re-circulation line from 1.5" to 2". That will be about 10.5 ft/s instead of 18 ft/s.

We have an ARC valve to control recyculation, and the temp. is about 225F and the pump discharge is 890PSIG, DA pressure about 5 psig.

 

[georgeverghese]

Whilst your suspicion that the DA inlet is creating excessive backpressure on the ARC may be right, the other criterion to meet when you use an ARC is that the pump Q-h curve should be fairly sloped at around the min recirc operating point - check that this is the case also just to be sure.

The other thing to check is that the sizing case for the ARC should account for localised flashing of this hot BFW stream at the superimposed built up backpressure.

 

[LittleInch]

If the liquid was very clean, non gaseous, the 44ft/sec (13 m/sec) is possibly OK, but much faster than you should have.

However with the high possibility of bubbles and possibly debris, then this is far too high.

There are erosion calculators around but the hardest part is putting a number to the cleanliness or amount of gas.

Any piping designer should recognize that your velocities are excessive. If they don't then it would make me suspicious of all their other designs and calculations....

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

 

[robertj9]

Thank you guys! I really appreciate your answers.