Leak point pressure

Fig. 16.5. Leak-point pressure estimation by modified VCU (Pves vesical pressure, Pu l urethral closure pressure...

Preservation of the upper urinary tract is the central goal in the treatment of neurogenic bladder in children with myelodysplasia. Predictive indicators for upper urinary-tract fate are detrusor sphincter dys-synergia, high bladder-filling pressure, poor bladder compliance, moderate to severe fibrosis of the detrusor (OzKAN et al. 2005), high leak-point pressure, and vesicoureteric reflux (Seki et al. 2004). 

Because there are neither flanges, nor packed or gasketed joints inside the shell, potential leak points are eliminated, making the design suitable for higher-pressure or potentially lethal/toxic service. However, because the tube bundle carmot be removed, the shellside of the exchanger (outside the tubes) can only be cleaned by chemical means. 

Pumps handling flammable materials represent a significant potential for spill and subsequent fire. This is due to damage to seals and failures of other potential leak points. The first consideration in fire protection for pumps is their location relative to other equipment, vessels, process structures and buildings housing personnel, and key control or utility systems. When locating a pump, consideration should be given to the size, properties of material handled, temperature, and pressure. 

Here indices T and N refer to the one or the other pressure ratio and indices 1 and 2 reference the inside and outside of the leak point, respectively. 

Here the points suspected of leaking at the pressurized test specimen (see Fig. 5.4, d) are carefully traced with a test gas probe which is connected with the leak detector by way of a hose. Either helium or hydrogen can be detected with the INFICON helium leak detectors. The sensitivity of the method and the accuracy of locating leaky points will depend on the nature of the sniffer used and the response time for the leak detector to which it is connected. In addition, it will depend on the speed at which the probe is passed by the leak points and the distance between the tip of the probe and the surface of the test specimen. The many parameters which play a part here make it more difficult to determine the leak rates quantitatively. Using sniffer processes it is possible, virtually independent of the type of gas, to detect leak rates of about 10 mbar l/s. The limitation of sensitivity in the detection of helium is due primarily to the helium in the atmosphere (see Chapter 9, Table VIII). In regard to quantitative measurements, the leak detector and sniffer unit will have to be calibrated together. Here the distance from the specimen and the tracing speed will have to be included in calibration, too. 

The bomb is then placed in the jacket, and the gas inlet tube is fastened securely. The thermocouple is also inserted. From this point on, all operations should be performed on the other side of the wall separating the operator from the equipment. Hydrogen is bled into the bomb from the high-pressure tank, the valve to the tank is closed, and the bomb is allowed to stand for about a half hour in order to test for leaks. If a leak has developed, as indicated by a drop in pressure, one locates it by painting all the joints with a soap solution and looking for bubbles of gas, which indicate a leak. The connector is tightened, and if this does not stop the leak, the pressure in the system should be released, the connection disassembled, and the tapered end of the tube re-formed with a tool supplied for the purpose. These measures usually will stop the leak, but if they do not, it will be necessary to ream out the connector seat with the reamer designed for this purpose. 

Fig. 6. Retain capacity, (a) Leak off pressures and repeat formation test data from the North Sea Central Graben. The curve is the minimum in situ horizontal stress trend, determined from the lower bound of leak off test data, (b) Retain capacity in the North Sea Central Graben. Retain capacity is the difference between the minimum horizontal in situ stress and the fluid pressure at any point. At low retain capacity (1000 psi or lower) the likelihood of trap failure is increased (Gaarenstroom et al., 1993).

DUMMY PISTON - The axial thrust in reaction turbines can be nearly eliminated by the use of balance or dummy pistons. With the correct size of a dummy piston exposed to two different bleed point pressures, the thrust is nearly equalized. There is a small leakage across the labyrinth seal of the dummy piston as steam leaks from the high to the lower bleed point... 

The mass flow rate from a rounded nozzle branch is greater than through a conventional pipe branch but it is the latter which is generally used. Further possibilities are leaks from drain and sample points, pressure relief devices, bursting discs, seals and flanges, and pipe ends. 

Tracer gas analysis. At relatively low system pressures, a small amount of an easily detected tracer gas (usually sulphur hexafluoride, SF6, or helium) is injected into the system to be tested, and leak points identified with a suitable detector. This method can be extremely sensitive and is the method of choice when the highest level of containment is specified. From the rate of tracer gas emission, the nominal dimensions of the leak can be measured (G. Leaver, 1990, personal communication), which is a first step in assessing the risk in biological terms. 

When a liquid is converted into a spray either deliberately by an atomiser or inadvertently from a small leak under pressure, the spray exhibits more of the characteristics of a gas than of a liquid and its flash point and ignition temperature etc. are consequently lower than the liquid values. The effect of converting a liquid into spray droplets is to increase the surface area of the liquid and hence the rate of evaporation. This in turn means that there is a greater concentration in the gaseous phase around the droplets than occurs over the bulk liquid and so combustion is more easily achieved. 

Seat leakage is specified for conventional direct spring operated metal-to-metal seated valves by API RP 527. The important factor in understanding the allowable seat leak is that it is stated at 90% of set point. Therefore, unless special seat lapping is specified or soft seat designs used, a valve operating with a 10% differential between operating and set pressures may be expected to leak. 

Locate leaks Apply soap solution to tape bleed holes and threaded connections depressure when tests are done. Block in N2 and check for loss of pressure. Check all connections. Same as in Step 1. Tag flanges to identify leaks tighten or repair leaks each time a system is pressurized, drain condensate at low-point bleeds. 

Refrigeration, like dilution, reduces the vapor pressure of the material being stored, reducing the driving force (pressure differential) for a leak to the outside environment. If possible, the hazardous material should be cooled to or below its atmospheric pressure boiling point. At this temperature, the rate of flow of a liquid leak will depend only on liquid head or pressure, with no contribution from vapor pressure. The flow through any hole in the vapor space will be small and will be limited to breathing and diffusion. 

Material stored at or below its atmospheric pressure boiling point has no superheat. Therefore there will be no initial flash of liquid to vapor in case of a leak. Vaporization will be controlled by the evaporation rate from the pool formed by the leak. This rate can be minimized by the design of the containment dike, for example, by minimizing the surface area of the liquid spilled into the dike area, or by using insulating concrete dike sides and floors. Because the spilled material is cold, vaporization from the pool will be further reduced. 

Small cocks have often been knocked open or have vibrated open. They should never be used as the sole isolation valve (and preferably not at all) on lines carrying hazardous materials, particularly flammable or toxic liquids, at pressures above their atmospheric boiling points (for example, liquefied flammable gases or most heat transfer oils v./hen hot). These liquids turn to vapor and spray when they leak and can spread long distances. 

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