IE41954B1 - A method of dampening surge pressures in a liquid system - Google Patents

Abstract

1535525 A method of dampening surge pressures in liquid flow lines TII CORP 26 Nov 1975 [9 Dec 1974] 48607/75 Heading B1C In a method of dampening surge pressures in flow lines the flow line 225 through which liquid is supplied is provided with an orifice plate 224 to cause turbulence in the liquid flow the plate causing a vena contracta portion in the liquid at a distance of 0.25 to 0.5 pipe diameter downstream of the orifice plate, the diameter of the orifice being from 0.7 to 0.9 of the diameter of the flow pipe. An amount of gas is then introduced into the flow pipe, the gas flowing through a nozzle 226 the tip 232 of which is located in the vena contracta portion so that gas is dispersed from the tip into the liquid in the flow pipe, the amount of introduced gas being in excess of that required to saturate the liquid and hence dampen the surge pressures thereof. The method is particularly directed to the reduction of water hammer in flow pipes.

Description

This invention relates to a method of dampening surge pressures in a liquid system and has particular, but not exclusive, application to the reduction of surge pressure or water hammer in a liquid flow line of a waste treatment system.

According to the invention there is provided a method Of dampening surge pressures, comprising:supplying liquid through a flow pipe, said flow pipe containing a turbulence causing means in the form of a fiat plate orifice, said flat plate orifice causing a downstream vena contracta portion in said liquid, said vena contracta portion being located at a distance Of from substantially 0.25 to substantially 0.5 pipe diameters downstream from said flat plate Orifice, the diameter of said orifice ranging from substantially 0.7 to substantially 0.9 of the diameter of said flow pipe; and introducing an amount of a gas into said flow pipe, said gas flowing through a nozzle of which the tip is located in said vena contracta portion so that said gas is dispersed from said tip into said liquid within said flow pipe, the amount of said introduced gas being in excess of that required to saturate said liquid by whioh to dampen surge pressures thereof.

In order that the invention may be well understood there will now be described an embodiment thereof, given by way of example, reference being had to the accompanying drawings, in which Figure 1 is a cross-sectional view of an injection mixing elbow: Figure 2 is a cross-sectional view of a flat plate orifice as used in the construction depicted in Figure 1; and Figure 3 is a cross-sectional view of a nozzle.

Force mains of a waste water treatment system, for example, operate intermittently according to the influent rate to the wet well and the level settings used to control the pumps. When the pumps shut down, a pressure wave travels through the system, is reflected, returns, and oscillates periodically ultimately damping out. The pressure fluctuations occur below and above the static pressure level in the line. The pressure differences may compare with the dynamicstatic pressure difference or they may exceed this difference. Such pressure wave surges are referred to as water hammer. Air present in force mains incident to aeration thereof to control septicity affects these pressure waves. The presence of air reduces the pressure differences, it reduces the velocity of the pressure waves in the force main, from one end to the other, and the air damps out the pressure oscillation rapidly in a reduced number of cycles, all in comparison to the force main response to pump shut down without air injection to the force main. All these results are beneficial and are a bonus accruing from the practice of air injection to force mains. Thus, force mains used for waste, water, or liquid generally, such as oil, may benefit from aeration, or inert gas injection as with engine or boiler exhaust gas, nitrogen or carbon dioxide. Preferred gases are those which are not unduly reactive and which exhibit low saturation levels in the 419 54 - 4 liquid transported. This reduces the gas compressor capacity required to inject an excess of gas beyond the saturation concentration. The beneficial results on pressure reduction occur predominantly from undissolved gas.

Water hammer or pressure wave surges can be abated or greatly reduced by injecting an amount of gas in excess of the saturation level of a gas in a liquid. Desirably, a large excess is preferred such as from 5 to 10 times the saturation level. Generally, excess of twice the saturated level is necessary to produce suitable results. To ensure that the saturation level is reached, the gas is injected at a highly turbulent region of flow in a liquid piping system suoh as a main Or transmission line as exemplified by a pipe.

A specific example of a high turbulence causing device containing a gas injector member is shown in Figure 1« An injection mixing elbow generally indicated by numeral 222 having a flat plate orifice 224 is located within the elbow at the commencement of the radius. A pipe 225 is attached to the elbow in any conventional manner. A small diameter or nozzle tube 226 is inserted through the elbow and through the orifice in the plate 224 so that the tip 232 is located within the high turbulence and at the vena contracta portion downstream of which full mixing occurs within the flow line or pipe 225.

The location of the tip 232 of the nozzle 226 is important with respect to. thorough mixing and suppression of concentration gradients. Thus, the tip 232 is located from substantially 0.25 to substantially 0.5 pipe diameters downstream of the flat plate orifice 224 or at a highly preferred distance of from substantially 0.36 to substantially 0.39 diameters with 0.375 diameters being the optimum location.

The flat plate orifice 224 as utilized in the elbow is shown in Figure 2. The orifice diameter is from substantially 0.7 to substantially 0.9 of the pipe diameter and may have a taper (at about 60°) leading from the orifice opening.

Although the flat plate orifice and the gas injector nozzle have been shown in an elbow they could equally as well be positioned in a union or straight pipe.

Another important aspect of the introduction of gas into the system by inserting more gas than that required to saturate the liquid downstream of the pump is that pump prime is not lost, whilst, as stated, pipe kock caused by pumping is significantly dampened.

Although the gas injecting nozzle may generally be a thin pipe or tube, a preferred nozzle is Shown in Figure 3 generally indicated by numeral 350. The nozzle 350 generally has a first portion having an average thickness indicated by numeral 352. The diameter of the nozzle in a second portion proportionally increases until a very thin annulus 353 exits at the tip generally indicated by the arrow 354 of the nozzle. The slope of the tapered portion of the nozzle is generally less than 7° and preferably about 2° to about 3°. A desirable thickness of the annulus at the tip of the nozzle is about 0.01 inches.

The diameter as indicated, generally increases at proportional rate to accommodate pressure drop of the gas and moreover to ensure good strength and rigidity of the nozzle portion. Such a nozzle also tends to reduce the flow of the gas. Due to the provision of a very thin annulus at the tip of the nozzle, the injected gas is in very close vicinity to the conduit fluid and thereby tends to reduce any eddies as normally encountered with thick walled nozzles. Moreover, additional shearing action is encountered due to the lack of eddies and thus promotes efficient and thorough mixing of the injected fluid in the vena contracta region of a turbulence causing device in the form of a flat plate orifice.

Preferably, one or more further turbulence causing devices such, as flat plate orifices are located downstream from the flat plate orifice 224 throughout the liquid flow system preferably separated by at least one transition length e.g. from 25 to 40 pipe diameters for turbulent flow and preferably more than 40 diameters to maintain the saturation level of the liquid. The amount and number Of such devices will depend largely upon the system utilized as should be apparent to one skilled in the art.

The net effect of the addition of an excess amount of gas above the saturation level of the liquid is to provide a distributed air chamber along the entire length of the flow pipe and, system which acts as a distributed surge suppression air chamber rather than a discrete air chamber. As above noted, preferred gases are those which are not unduly reactive and whioh exhibit low saturation levels in the particular liquid transported. Of course, numerous gases may be utilized. Specific preferred gases generally include: oxygen, ozone preferably in a carrier gas, e.g. air, nitrogen, etc; nitrogen; carbon dioxide, air, natural gas, exhaust gas, e.g. from a diesel engine driven pump, distillate gases such as propane, butane, pentane, etc. and the like.

Although the above described surge suppression system may be utilized in generally any liquid flow system, it has been found to be particularly suitable in suppressing surge pressures in any liquid transmission system such as 4195 the flow system of a waste treatment facility and may be added in the force mains.

Attention is drawn to Patent Specification No. y// , and Patent Specification No. v / ''both of which have been divided out from the present application.

Claims (9)

1. CLAIMS:1. A method of dampening surge pressures, comprising: supplying liquid through a flow pipe, said flow pipe containing a turbulence causing means in the form of a flat plate orifice, said flat plate orifice causing a downstream vena contracta portion in said liquid, said vena contracta portion being looated at a distance of from substantially 0.25 to substantially 0.5 pipe diameters downstream from said flat plate orifice the diameter of said orifice ranging from substantially 0.7 to substantially 0.9 of the diameter of said flow pipe; and introducing an· amount of a gas into said flow pipe, said gas flowing through a nozzle of which the tip is located in said vena contracta portion so that said gas is dispersed from said tip into said liquid within said flow pipe, the amount of said introduced gas being in excess of that required to saturate said liquid by which to dampen surge pressures thereof,

2. A method according to claim 1, wherein said flow pipe contains at least one turbulence causing device located downstream from said flat plate orifice, said downstream device also being a flat plate orifice.

3. - A method according to claim 2, wherein the or each downstream turbulence causing device is located at least 40 pipe diameters downstream from said firstmentioned flat plate orifice.

4. A method according to any of the preceding claims, wherein the location of said nozzle tip is from 0.36 to 0.39 pipe diameters downstream from said flat plate orifice.

5. A method according to claim 4, wherein said nozzle tip is located at 0.375 diameters downstream from said flat plate orifice.

6. A method according to any of the preceding claims, wherein said injected gas is selected from the class consist ing of oxygen, ozone in a carrier gas, nitrogen, carbon, dioxide, air, natural gas, exhaust gas and distillate gas. 5

7. A method according to any of the preceding claims, wherein said carrier gas is selected from the class consisting of air, oxygen enriched air, and oxygen.

8. A method of dampening surge pressures as claimed in olaim 1 and substantially as herein described with

9. 10 reference to the accompanying drawings.