Weep Point

At this point it is probable that in time even a more elastic liner will be damaged and allow water or whatever else the pipe is carrying to ooze or weep through the wall. As is the case with the yield point of steel pipe, reaching the weep point is not cataclysmic. The pipe can still continue to withstand quite a bit of additional load before it reaches the point of ultimate strain and failure. Recognize that a more substantial, stronger liner can easily extend the weep point. 

The weep point or strain-to-first-crack in a wall for filament-wound pipe constructed using isophthalic plastic is currently found to be not less than 0.009 in./in. This has been repeatedly demonstrated by careful coupon testing and burst testing of pipes with strain gauge instrumentation attached. 

design values are based on the strain-to-first-crack or the empirical weep point. For normal design conditions a strain of 0.0018 in./in. is used, which provides a five-to-one safety factor. For transient design conditions a strain of 0.0030 in./in. is used, for a safety factor of approximately three to one. To those familiar with the design safety fac- 

The lower limit of the operating range occurs when liquid leakage through the plate holes becomes excessive. This is known as the weep point. The vapour velocity at the weep point is the minimum value for stable operation. The hole area must be chosen so that at the lowest operating rate the vapour flow velocity is still well above the weep point. 

Several correlations have been proposed for predicting the vapour velocity at the weep point see Chase (1967). That given by Eduljee (1959) is one of the simplest to use, and has been shown to be reliable. 

The clear liquid depth is equal to the height of the weir hw plus the depth of the crest of liquid over the weir how this is discussed in the next section. 

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The weep point is considered to be the minimum vapor velocity that will provide a stable tray operation, pre-... 

Hsieh and McNulty [210] developed a new correlation for weeping of sieve and valve trays based on experimental research and published data. For sieve trays the estimation of the weeping rate and weep point is recommended using a two-phase countercurrent flow limitation model, CCFL. 

The weep point for sieve or valve trays is the vapor rate at which the liquid weeping rate is diminished to zero. Thus, J L approaches zero asJ G is increased [210]. For a vapor rate that leads to J g higher than the weep point value, then there should be no weeping. 

The higher the value of W ndex> the more confidence that there will be no weeping [210]. At a constant weep point, J G then, the higher the percentage opening of the tray, and the higher will be the vapor volumetric flow required to satisfy the weep point criteria. 

To calculate the weep point, use J g = 0.74 and calculate Z from (a) above, then calculate Vg from (e) above. 

The weeping rate of the sieve tray is strongly influenced by the gas flow rate, that is, the weeping rate will increase as the gas flow rate reduces below the weep point, i.e., where the weeping starts. Note the comparison of sieve and valve trays during weeping, Figure 8-135 [210]. 

This is based on the correlation of Mayfield [45] where hjt (weep) = dry tray pressure drop at tray weep point, in. liquid. 

A. Select a design velocity near the weep point if ... 

Reduction in efficiency can be tolerated if vapor rate falls to weep point minimum or below. 

Read Figure 8-130 effective head = 1.58 in. liquid Total Wet Tray Pressure Drop ht = 0.608 + 1.58 = 2.188 in. liquid Weep Point... 

The selected design Fj = 17 gives the number of holes to operate at these conditions. Note that the values of 1223 and 1410 holes for the top and bottom respectively indicated operations somewhat closer to the tower maximum than to the weep point. This usually insures as good an efficiency as is obtainable for a given system. It may limit the flexibility of the tower, since there will not be enough holes to operate down to the weep point at the given design flow rates. 

Reading the intersection of 3.45 vs. 2.08 shows that for either weep point curve, the weep point is well below the ralues for operation, so this design not near the weep point... 

By trial and error the tray can be examined to determine the rates that will coincide with the weep point. Thus, the entrainment can establish the upper limit of operation, and the liquid weeping through the perforations represents the lower limit of stable operations that is, turndoAvn is generally used to represent the ratio of the... 

Minimum velocity through holes at weep point, ft/sec... 

So minimum operating rate will be well above weep point. 

Weep point correlations for valve trays were presented by Bolles (loc. cit.) and by Klein (Chem. Eng., Sept. 17,1984, p. 128). Hsieh and McNulty (loc. cit.) gave a complex extension of their weep rate correlation to valve trays. 

Dumping As gas velocity is lowered below the weep point, the fraction of liquid weeping increases until all the liquid fed to the tray weeps through the holes and none reaches the downcomer. This is the dump point, or the seal point. The dump point is well below the range of acceptable operation of distillation trays. Below the dump point, tray efficiency is slashed, and mass transfer is extremely poor. Operation below the dump point can be accompanied by severe hydraulic instability due to unsealing of downcomers. 

As vapor rate is lowered, either at constant liquid rate or at a constant L/V"ratio, the limit of excessive weeping is reached. This limit is not identical with the weep point, as some weeping can usually be tolerated. 

The weep point is defined as the vapor rate when weeping first becomes noticeable. At that point, little efficiency is lost. As vapor rate is reduced below the weep point, the fraction of tray liquid falling through the holes increases, and the reduction in efficiency becomes more noticeable. When this fraction is sufficiently large to effect a sig-... 

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