The following article was written by Alan Sunudo of A.R.I. Flow Mexico and republished here with permission.
It’s well known that pumps and pipelines are our most valuable assets in fluid transmission systems. So why not choose the right devices to protect those components against abrupt changes in velocity, thus in pressure than can lead to great losses?
Surges in pressure can be caused by several scenarios. The most dangerous of these scenarios can be a pump trip due to an interruption of energy supply in pumping stations, amongst others, such as pump start up or sudden closure of a downstream valve in a gravity system. Within these scenarios we can have water hammer, cavitation, water column separation, collapse, etc.
These are subjects that we can investigate about, but now we will focus on a system that has been recently designed by A.R.I. Flow Mexico as a subsidiary of A.R.I. Flow Control (Israel) and now we are supplying all of the devices that will be explained in this article. The system will be initiating at the end of 2020, and it’s located in the north of Mexico, Sonora more precisely.
In the applications engineering department we have developed the modelling/sizing of all the surge protection devices that will be installed along the pumping stations and pipeline. Let’s talk about system specs and profile:
As you can see, the profile consists in 4 stages of pumping. Each pumping manifold of each PS has its system curve and all the modelling was made with a specific flowrate-head curve. After setting up the model in KYPipe, the first scenario to calculate and adjust is steady state flow. Which we can see in the image above.
Afterwards we can proceed to calculate unsteady state scenario (or surge) by inducing a pump trip to each pumping station and obtaining maximum and minumum pressure envelopes as described below. Upsurges and downsurges in pressures on each PS were as high as 25 bar and as low as -10 mH20 which is the limit for cavitation at a water column separation when not mitigated,
According to the results of the unsteady state and non-protected system, we proceed to propose and size the corresponding devices. We all know that every system has to be iniatially filled and at some point of its operational life, discharged. So we necessarily have to include air valves in our analysis. But be careful, the wrong sizing of an air valve orifice when it will be operating in such a critical scenario, can be more dangerous that we think, and sooner or later can cause additional upsurge problems because of premature closure.
With that said, it’s important to double check on air valve sizing when using them as surge protection device. Air valves can help in great manner when they’re well sized, but it’s not a good practice to use them as the only solution for unsteady state scenarios. Air valves must have a mechanism that slowers the air discharge when in an upsurge and allows complete air intake. That’s also known as a Non-Slam air valve. It’s also important to mention that the system described in the boundary conditions table, will pump reclaimed water, so surge protection devices will have to work under such fluid conditions. In the picture above we can see our D26 NS model that is fabricated and designed to withstand reclaimed water that will have suspended solids.
Following Delft University recommendations depending of which is the performance criteria violation we’ve used Bladder tanks in each pumping station. Of course, after revising other devices such as SAV (Surge anticipating valve) or PRV (Pressure relief valve), but in this case we did not recommended them because of the poor maintenance that the system will have.
After modelling the system with Non-Slam air valves and Bladder Tanks, we’ve obtained the pressure envelopes shown in the next figure. Were we can observe that there’s no considerable upsurge (it has been reduced down to 40%) , just as there’s no negative pressures that may expose the pipeline to collapse due to maximum downsurges.
The final solution consisted of 4 bladder tanks (15, 5, 18 and 5 cubic meters of volume respectively for each PS) and 48 air valves (3 and 4″) along the pipeline located in critical points were the AFP (Adimensional Flowrate Parameter) indicated the air bubbles could stay in such points and would be dragged by fluid because of its velocity and pipe slope of the studied section.
Bladder tanks are always a great solution, because of their ease in maintenance and robust performance under upsurge and downsurge. In the world of surge protection devices, not every one of them can work on both sides of the transient. And A.R.I. can provide solutions that combine every and each of the devices mentioned in this article. In this case, when combining Bladder Tanks and Air Valves, the size of the tanks is always reduced in a considerable manner. Because air valves help the tank when downsurge is occuring, so we would have water entering the system from the tank, and air intake to the system as well.
With this solution, client was extremely satisfied, they’re now under construction and air valves just arrived in the past few days. We are now expecting the tanks to arrive as well and make the initial start-up of this complex system soon. Unfortunately we are not allowed to publish any pictures of the system yet. But we will as soon as it’s finished.
A.R.I. is always proud to announce projects like this one all over the world. If you have any questions regarding this subject you can contact me at alan.sanudo@ari.mx
