A CONTROL SYSTEM FOR MAINTAINING THE PRESSURE IN A CLOSED PIPING SYSTEM

Abstract: Closed piping systems for liquids require pressure maintaining arrangements to define the level of pressure in the piping system in relation to the surrounding atmosphere. Various values of heights of delivery along the piping system are measured depending on at which point in the piping system the pressure maintaining arrangement is connected to it. In particular, different values are measured at partial load. By connecting the pressure maintaining system to the interior of the pump a more or less symmetric pressure picture is created. In this way a minimal risk of overload of the piping system is achieved, be it under stationary or transient conditions.

Description: A control system for maintaining the pressure in a closed piping system. Background Closed piping systems for liquids require pressure maintaining arrangements to define the level of pressure in relation to the environment (most- often the atmosphere). Ordinary methods of maintaining the pressure include expansion vessels or pressure maintaining pumps. In an expansion vessel the pressu¬ re maintaining pressure is controlled by the pressure in a gas cushion and when using pressure maintaining pumps the lifting height define the pressure maintaining pressure one way or the other. The pressure maintaining equipment must be able to handle va¬ riations of the volume of the enclosed liquid, i.e. to feed or to dispose of liquid from the closed system so that the pressure level can be kept almost unchanged independent -of the volume of liquid. Changes of the volume will appear depending on changes of temperature on pressure and includes also changes of volume in the armatures of the piping system. The pressure maintaining pressure is chosen so that the inte¬ rior overpressure of the piping system does not exceed the construction pressure of the system at any point and so that it at the same time exceeds the current steam pressure. Depending on where the pressure maintaining arrangement is connected to the pipes of the system different values of the pressure are measured. Apart from the position of the connec¬ ting point in the system the pressure picture is influenced by the geometry of the piping system, the properties of its components, methods of regulation and flow. The pressure maintaining arrangement is usually connected to the system near the pumps on its low side or compression face depending on which pressure picture is wanted. The preferred pressure picture depends on the way the pipings of the system are placed. In big piping systems like e.g. district heating networks the topography of the landscape decides the location of the pipings, in industrial systems the lay out of the pro¬ cess plant is decisive, in houses the plan of house is an im¬ portant factor and in flow technical devices an optimum of size and costs is of importance. According to the present invention the pressure maintaining arrangement is connected to an interior part of the circula¬ tion pump of the closed system, i.e. in the middle of the pressure picture or. somewhat displaced. In most cases this will be the most optimal method. The lines of height of delivery in figures la-c illustrate the importance of the connection of the pressure maintaining sys¬ tem for some different operation situations. The term height of delivery refers to the sum of the internal overpressure in the piping system measured in meters liquid column and the le¬ vel in meters over a selected horizontal level or with symbols H - . + Z Sg where H = height of delivery M/C P = internal overpressure Pa S = density of liquid kg/m-3 g = acceleration of gravity m/s^ (9.81) Z = level (plusheight) m A line of height of delivery is consituated as a function of X, where X " is a coordinat along the pipe line. Characteristics The most distinctive characterizing part of the invention is that the pressure maintaining arrangement is connected to an internal part of a pump in order to obtain a suitable pressure level in the piping system. In a pump the pressure is higher at the compression side than at the low side. By chosing a conve¬ nient point of connection almost any intermediary pressure can be achieved. Often the "middle pressure" is convenient to connect to the pressure maintaining arrangement. Figure 2 shows an example of a connection where the "middle pressure" is aimed at. The pressure maintaining equipment (1) is con¬ nected to the interior of the pump (3) via a connection line (2) and will thereby control the pressure level in the pipes (4) of the closed system. There are many ways to displace the pressure picture of the piping system at a given pressure maintaining pressure, when the connection is done to an interior part of the pump e.g. : - Displacement of the position of the connection radially. - Provision of the pump wheel with double sealing edges with with intermediary connection with the blade channels. - Use of back blade to adapt the pressure at the connecting point. - Connection of the pressure maintaining equipment to open wheels via the wearing, disc. - Connection of the pressure maintaining equipment to one of the transfer ports of multi-stage pumps. The connection between the pressure maintaining equipment and the pump must have enough size to take up maximum required pressure maintaining flow. This is decided by maximum change of volume per time unit for the enclosed mass of liquid, i.e. of maximum pressure- and temperature changes per time unit and of the volume of the system. A compensation for leakage has to be added. Method of adapting the capacity of the connection to the demands of the system are for instance: - Changing the dimension of the individual connection. - Changing the number of connections. - Making double sided connections. - Pressureregulati ' on of the pressure maintaining pump accor¬ ding to a preferred internal overpressure. Technical effect A pressure maintaining system "in the middle of the pump" offers a number of advantageous technical effects associated with the delivery height picture under normal operation conditions and in relation to pressure transiant when starting or stopping the pumps. ^ Since the pressure maintaining system is "in the middle of" delivery height picture of the pipe network the maximum case, i.e. the dimensioning flow, to be dimensioning also in rela¬ tion to the pressure; At partial load the delivery height pic¬ ture will shrink towards the middle and forward lines as well as return lines will go away from their limits (maximum allow¬ ed internal over pressure resp. current steam pressure). Maxi¬ mum marginals against pressure transients are created auto¬ matically at all operational situations. The same effects can mainly be accomplished under normal ope¬ ration by connecting pressure maintaining equipment to "the most distant point in the network". The advantage of having the pressure maintaining equipment at one place will then be lost, i.e. the place where -the ' operators will normally be. "The most distant point" may ' moreover be different points at different operation situations. A pumpstop, particularly the uncontrolled pumpstop when the current is suddenly broken, is a quick " change of the strea- mingcondition in the piping system and creates pressure tran¬ sients. To express it in a simplified manner a pressure wave starts from the pump with an amplitude of the same size as the lifting height of the pump. The pressure wave spreads in the piping system at a rate of 1100 m/s (water is steel tubes) and changes the condition in all connected pipe parts. This is the fastest pressure and volume change that can come in question in a piping system. Maintaining pressure "in the middle of the pump" will divide the strength of the pressure wave, will result in a pressure- lowering wave in the forward line and a pressure increasing wave in the return line. Since the forward- and return lines in a closed piping system have the same length, the two waves will extinguish each other in an optimum manner. If the pump rolls out in a closed system with pressure main¬ tenance before or after the pump the highest and the lowest pressure will normally appear at the low side resp. compres¬ sion face of the pump some seconds after loss of driving effect. This risk of overload is eliminated having the pres¬ sure maintaining equipment "in the middle of the pump". The height of delivery picture of the network will collaps "inwardly" with a minimal risk of interferense between pres¬ sure waves from branch lines and thus also a minimum risk of unallowed pressures. Analogously corresponding favourable effects are achieved at pump start. Since the pressure picture in the piping network is changed symmetrically at a quick pumpoperation 'the net change of volu¬ me of the enclosed liquid will be at a minimum, i.e. the pres¬ sure maintaining equipment will have maximum possibilities of taking up other changes of volume caused by variations of the temperature or leakage. The principal technical effect of the invention is thus that an installed equipment in a closed system regarding pressure can be used in an optimum manner at all flows, that the risks of unallowed pressures caused by pressure transients in con¬ nection with start/stop of pumps is practically eliminated and that optimum marginals against other pressure transients is created automatically e.g. at sectioning.

Claims: Claims 1. At control system for maintaining pressure in closed piping systems for liquids having a circulation pump whereto an ex- pansionvessel, pressure maintaining pumps or another known pressure maintaining equipment is connected characterized in that the connecting "stream channel (channels) (2) between the pressure maintaining equipment (1) and the piping system (4) is connected to the interior of the circulation pump (3) in such a way that pressure maintaining pressure is higher than the pressure at the low side of the pump and lower than the pressure at the compression face of the pump. 2. A system as claimed in claim 1 characterized in that the cir¬ culation pump is a 1-stage or a mutiple stage centrifugal pump. 3. A system as claimed in claims 1 and 2 characterized in that the connecting streamchannel (channels) is connected to the circulation pump at a radial distance from the rotation axis of the pump wheel, that is greater than the distance to the outer edge of the pump axis and less than the distance to the outer edge of the pump wheel.

Inventors: GUSTAFSON BROR-ARNE, GUSTAFSON, BROR-ARNE
Applicants: FLUDEX AB
Application year: 1985
Main classification: F04D15
IPC codes: F04D15