Winterisation flows ?, typical values used. 1

[mrtangent]

Dear Sirs,

I'm curious as to what other people do on winterisation flows and how much. I've seen some guideance which states for

3 inch pipe 0.02 m3/hr
above 3 inch 0.1 m3/hr

These seemed very low flows to me.

These are much lower than our winteristaion orfice plates on exchangers are sized for. The plant designers used 0.5 m3/hr for all winterisation lines - ie much higher !. We are in the UK and on the coast so it does'nt get abornally cold here.

The reason I'm asking is we pay a premimum for our water usage we can significant cut our costs by reducing our usage.

We have several scrubbers(A lot) which send to our effluent plant "sometimes" but require winterisation flows when they are not taking vents. I dont want to set these flows to high.

As ever thanks in advance for peoples advice.

 

[sshep]

The guidence which you have been given is incomplete in that the length of pipe being protected is not considered. Bleeding water to prevent freeze only works because "warm" make-up is constantly entering the pipe to replace the cold discharge from the end. The idea that just moving water will keep it from freezing is a myth.

The design flow of 0.5m3/hr (2.2 gpm) does seem on the high side as a general rule, but when using an orifice there is a practical issue of orifice size- a small orifice can easily plug. If you replace these consider installing a wye strainer upstream to protect the smaller bore.

I have seen several nomographs over the years for the freeze calc which is conceptually easy to understand. A complete solution requires the pipe thickness and thermal conductivity (including insulation if present) and other data for calculating U, the outside and make-up water temperature, and heat transfer area (i.e. calculated from pipe diameter and length). Estimating the outside resistance for the U calc is especially tough because it is situation specific- i.e. exposure to wind, etc.

On a practical level you can get freeze protection traps from several steam trap companies (spirex-sarco, etc) which automatically sense and bleed off cold water as needed from the end of the pipe. If you use these follow the recommended installation guidence (proper location, upstream strainer, etc). A maintenance program is required because these devices can open too soon (typically) or too late after a few years of service.

 

[mrtangent]

sshep,

My appolgies, I forgot the rates were for 50 meters of pipe. For each extra 50 meters the guide was to pro-rate up the flows.

 

[sshep]

Thanks for the clarification.

I found one of those nomographs in vol 07-2 of Chemical Engineering's Calculation and Shortcut Deskbook. It was a weaker and less documented basis than some than I remember (-20F outside temp, 15mph wind, 40F make-up). Sorry about the units. Advice was given to double flow if wind velocity was >15mph (miles per hour). The following gallon per minute flow @ nom pipe diameter were given per 100ft of pipe: 1gpm@0"; 2gpm@2"; 3gpm@4"; 4gpm@6"; 6gpm@10"; 8gpm@14"; 10gpm@10".

In our Gulf Coast plants we use automatic bleed valves, but manually bleed extra flow on critical lines and during hard freezes. Water use is not as much an issue as the load on our waste water treatment plant.

 

[mrtangent]

Thanks for the advice.

My main reasonis because our water supply is a high cost item. Especially in the summer when we cannot use river water (or are very limited) and have to use welll water.

 

[TD2K]

Some companies offer freeze protection valves that I've seen. Essentially, they open using a wax plug or other similar device at just above freezing and open/close as the water temperature varies.

If you want to conserve water, that might be the item you want. I don't have any experience with them so I can't recommend specific manufacturers to you.