design of additive seal leg

[coold]

Can someone please explain to me how 'additive' seal legs work?

I have a seal leg, which must be designed to relieve pressure above 80kPag.

Rather than using only 1 water filled seal leg 8 metres high, I have been told that I can use 4 seals in series - each approximately 2 metres high (due to height restrictions).

Approx 1USGPM of seal water is fed continually to the bottom of the first seal, where it flows upwards in a 1" pipe, to the top where it flows horizontally in a 1" pipe, and then downwards in a 4" pipe, to the bottom where it flows into the bottom of the second seal. The second seal has the same dimensions as the first except the top is 8” lower. The top of each seal is 8” lower than the one preceding it, but the bottom of each seal is at the same elevation.

Each riser pipe has a blind flange at the top and bottom, except for the fourth, which has a gooseneck vent.

I am having a difficult time visualizing how the 2m high seals are able to be added to seal 8m. Any guidance would be greatly appreciated.

 

[DLANDISSR]

cool:

If the first seal is 2M and the top of each other seal is 8" lower than the previous one and the bottom is the same eleveation then each seal would be shorter than the previous one. This in itself would not add up to 8M.

If the system is totally full of water the system you describe will not work because the energy to raise the colum 2M would be gained back when the flow goes back down to the next column. The net effect of each column would be 2M plus the difference 8" between each column. Since 2M is about 37.4" and the three 8" drops would be 24" the total head would be approx. 2 x 37.4 = 24 = 50.8"

Keep in mind any energy used to raise a column on the up side would be gained back on the down side.

Draw the system and look at the total elevation difference between the inlet and the highest point minus the difference the outlet. That will be the head.

It will work if the down side 4" pipes are not full of water. Since there is little pressure gained if the down side leg if full of air. The pressure in each up side leg would be additive. Viz. The presusre in leg 1 at the outlet
would be 2m at the bottom. Since there is no pressure aditive for the column of air back up to the top of the previous column the pressure at the previous column top must also be 2M. The pressure at the bottom of the second column will have to be 4m (assuming each column is 2m tall)
and so on till at the bottom of the last column (closest to the source pressure)the pressure must be 8M.

If each of the columns is stepped down 8" then you must start with a 2.3 meter tall column and the last would be 1.7M tall. This would total 8M.

There must be some water left in the bottom of each 4" column so the trapped air will not bleed through.

I'm not sure what the 8" drop is for since the pressure will cause the flow. Gravity is not an issue in the flow through the system. The 8" drop can't hurt.

If the pressure is more that 8M the air will bleed through. If the pressure is less than 8M the water level will rise in the 4" columns to compensate. In any case the pressure limit will be 8M which cannot be exceeded with the given geometry.

I assume the outlet of this seal is gravity drained. There should be an air gap between the outlet of the seal and the drain pipe to assure that there will be no back pressure from the drain influencing the seal.