hydraulic grade line 2

[Thuba]

Good day, l have a hydraulic grade line, but l dont understand why it deeps down where there a valve, but pressure loss in a valve in not that much.

 

[1503-44]

There can be an advantage under certain startup and stop cases. If Pinlet is relatively close to pipe max design pressure at the inlet, a low flow may not have enough friction pressure drop to keep P below max all the way to the outlet, whereas full design flow would have a greater friction drop and keep P below Pmax everywhere. A low flow with backpressure at the outlet to keep away from column separation at the inlet may result in a very high operating pressure at the outlet, whereas open channel flow might arrive at the outlet with little to no higher pressure at all. The last segment of the BTC pipeline has a 30" diameter, as opposed to its normal 42" to increase friction drop at all flows and at times of very low flows, operates as an open channel, because of a large drop in elevation over the last segment. At low flow, the gain in pressure from decreasing elevation is very high in relation to low flow frictional loses. At high flow rates they pretty much cancel out.

 

[swazimatt]

On the topic example, how would you be sure that you will not be pumping a slug of air through the system on startup? Would controlling the startup flow to match the air valve discharge rate for a certain period may be the solution?

I imagine if the air is compressed enough it could cause some damage on the outlet