Siphon intake pipe in desalinization plants - examples

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We are designing an intake pipe for a desalinization plant that works as a siphon (pipe above HAT, negative pressure). This solution is often used and we have designed siphon intake pipes in some plants, However, our final client asks us for more examples beyond our experience. Could you provide me some examples?

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5 Answers

  1. Dear all,

    Thanks for the given information. However, I am more interested in a list of examples of intake pipes above high tide working as a siphon (to give to our final client) than the problematic issues that should be considered in design phase.

  2. Dear Mr. Victor Martinez, A very careful evaluation is required to avoid hogging of the siphon pipe system which can considerably reduce output efficiency. I am not sure of the capacity. However I have enclosed some concepts and sketches and hope it would be of interest to you. Write to me if need any further information.

  4. Dear Victor Martinez, Problems with seawater intakes and corrosion are the two primary causes for unscheduled downtime in desalination plants. On the surface, the design of seawater intake systems appears relatively easy. However, the dynamic, ever-changing characteristics of the sea, shoreline, and sea bottom present a variety of problems that must be considered when designing an intake system. One of the most important aspects of pump basin is the removal of "trash" from the water. Trash not only refers to solid materials such as driftwood and plastic containers, but also marine plants and animals which can find their way into the intake system. Water temperature also varies with depth. The deep oceans, this can result in thermoclines of 20 to 30°C between the surface and the bottom. Because of the rapid attenuation of radiant energy with depth, thermoclines of several degrees centigrade can exist between the surface and a depth of several meters. The magnitude of this surface thermocline is affected by the degree of mixing caused by waves. In shallow, protected bays, where there is limited heat transfer to cooler deeper waters and where wave-induced mixing is restricted, surface water temperatures may be several degrees centigrade higher than in open water. The seabed conditions will be one of the primary factors determining the types of sea water intake structure for particular location. The oceans are extremely dynamic systems constantly in motion as a result of external forces, such as wind and internal forces, such as temperature and salinity gradients. Seawater intake facilities are basically classified into three types, 1. Including beach well intake 2. Subsurface water intake and 3. Open sea water intake. This article considers the seawater intake technology options available for desalination plants, and will review the technologies employed to provide a reliable quantity of seawater at the best quality available. 1. Beach Well Intake- Beach well intake means that seawater out from intake well-constructed as close to the coastline as possible is filtered by seabed as the water source of desalination plant. This kind of seawater is especially attracted by seawater desalination reverse osmosis plant with the advantages of low turbidity and good quality due to particles in sea water is trapped from natural filtration. Seawater desalination intake project is the vital part of desalination plant, which is to ensure that sufficient, consistent and reliable feed water can be delivered for seawater desalination plant within the entire lifetime. The intake facilities and construction of intake structure both have important impact on seawater desalination plant investment, 2. Subsurface Water Intake- The deep seawater of open ocean is delivered to the shore-side through building intake pipes and then supplied to desalination project by pump plant constructed in the coastline, this is called” subsurface water intake”. Generally speaking, when sea level below 1~6m, intake water contains small fish, water grass, seaweed and other microorganism, thus water quality is not good. The content of these materials is 20 times reduction when intake level is above beneath sea level 35m, and water quality is good, which can greatly reduce the operating cost of the pre-treatment equipment. Meanwhile, water temperature in subsurface is lower, benefit to thermal seawater desalination process. This kind of intake is usually adopted on areas where seabed is quite precipitous, which is within 50 m from the coast and water depth up to 35 m. 3. Open Sea Water Intake Open sea water intake is the most common intake project. Although its water quality not good, it is still widely adopted due to advantages of small investment, wide application range and full application experience. siphon system that we would like to design is not a gravity line, since the driving force of flow is level difference. The system is also provided with vacuum pump to suck air out of the system during start-up. Once the water flows, it will be continuous (depends on head difference ) unless there will be siphon break. Line frictional pressure drop is proportional to square of velocity. Higher velocity means higher pressure drop and you may not be able to get the desired flow rate if the differential head is limited. Head differential is the difference between sea surface elevation and you destination pond elevation. The difference in head is due to the difference in levels between the sea and the basin, and this is what constitutes gravity flow - when liquid flows from a source at a higher level to a destination at a lower level without the action of an external driver such as a pump. The way you have described removing the air with a vacuum pump is fine - it is often done that way. It will work as long as both ends of the line are always submerged, or you have valves at the end of the unsubmerged line that can prevent air being sucked in while the line is being evacuated. This is called "initiating the siphon". The first is that you would have to use your vacuum pump to remove every last bit of air in order to get the full capacity of the pipe, otherwise you will have a permanent bubble sitting in the horizontal section restricting the flow of the water. You have shown your "horizontal" section as slightly sloped - this is probably a good thing to do, but remember to connect your vacuum pump to the highest point or you will always leave a bubble in the pipe. The second scenario is that in operation any air that enters the system will be trapped. Air can be released from the water when it gets to the top point because the pressure there is below atmospheric. Also because the pressure is below atmospheric it is possible to leak air into the pipe at flanges and fittings. If you ensure the correct Froude number any air that enters the line will simply be flushed out. If not, the siphon will eventually break or you will have to keep your vacuum pump running. The Froude number requirement is usually not difficult to meet. In your case with 3900 m3/h flowing in a 32" line you will have a velocity of 2 m/s and the Froude number will be about 0.7. This is certainly high enough to move any bubbles along the horizontal section, but it would be nice to have it a little bit higher in the final vertical drop into the basin. If you have a well-constructed line with minimal leaks the 32" line will probably be OK and will get rid of small amounts of air even in the vertical section. You will need a level difference of about 900 mm between the sea and the basin. If it is more than this your flow will be higher than 3900 m3/h and then you will definitely flush out any air. You can use following equation: ∆H = 2f (L/D).V^2/g Where f = fanning friction factor (dimensionless) ∆H = elevation differential (m) L = equivalent length - pipe + fittings (m) D = diameter (m) V= velocity (m/s) If you know the elevation of the top section, you can calculate the vacuum (pressure) using Bernoulli equation that includes frictional pressure drop terms. P1 + Z1 = P2 + Z2 + ∆-P (friction) Where 1 refers to top section and 2 refers to basin level. P2 is atmospheric pressure (1.013 bar a)