Oversized valves

Thermal relief valves are small, usually liquid relief valves designed for very small flows on incompressible fluids. They open in some proportion of the overpressure. Thermal expansion during the process only produces very small flows, and the array of orifices in thermal relief valves is usually under the API-lettered orifices, with a maximum orifice D or E. It is, however, recommended to use a standard thermal relief orifice (e.g. 0.049in2). Oversizing SRVs is never recommended since they will flow too much too short, which in turn will make them close too fast without evacuating the pressure. This will result in chattering of the oversized valve and possible water hammer in liquid applications. 

To size the SRV, calculate the minimum area necessary in order to flow the required flow. When selecting the SRV, choose the next API orifice letter up in practice, this is always a safety factor with normal API valves. Nevertheless, some manufacturers can customize valves to the exact required flow area (A). Oversizing valves is not good practice. Undersizing valves is simply dangerous. 

Uniquely, chlorine valves have their own oversize thread series 0 threads oversize, 4 threads oversize, 8V2 threads oversize, 14 threads oversize, and 28 threads oversize. Respectively, these are known as y4-14NGT (Cl)-l, Y4-14NGT (Cl)-2, y4-14NGT (Cl)-3, y4-14NGT (Cl)-4, and y4-14NGT (Cl)-5 threads. The (Cl) designation signifies chlorine. The first thread on a 4-thread oversize valve is the same size as the fourth thread on a standard 0-threads oversize valve. 

Oversize valves - For uses other than chlorine, oversize threads for revalving are generally but not always at 4 or 7 turns oversize. For chlorine, the 3/4 -14NGT(CI)-1 is not oversize the -2 is 4 turns oversize the -3 Is 8 1/2 turns oversize, the -4 is 14 turns oversize and the -5 is 28 turns oversize. 

In the chlorine application, oversize thread specifications apply only to the external valve thread. There are no oversize internal chlorine thread specifications for cylinders because the oversize valves are only used in conjunction with worn, out of specification, oversize cylinder threads. 

If the bore in the cylinder exceeds the maximum specified as in Table 8-2, it can result in flat crests at the point where sealing is supposed to occur. It is therefore important that the bore not be oversize and that a good bore gauge that registers off the cylinder thread crests at a depth of (L9) be used, as shown in Fig. 8-9. The first thread on a 4-thread oversize valve is the same size as the fourth thread on a standard valve, i.e., zero threads oversize. 

Design equipment to prevent excessively fast feed. Do not oversize pumps or control valves... 

Caution should be taken to prevent excessive acceleration. One solution is to provide for opening of the compressor discharge vent valve to increase the compressor flow and increase blower horsepower. This is effective with a centrifugal compressor, however, not with an axial compressor unless the compressor is provided with adjustable stator vanes that are reliable. This can be seen from the performance curve. The head versus flow curve for a given vane setting is extremely steep and opening the vent valve is ineffective. However, if the vanes operate fully open and the vent valve opens, the combined effect is satisfactory. The vent valve must not be oversized. 

An oversized relief valve may also chatter since the valve may quickly relieve enough contained fluid to allow the vessel pressure to momentarily fall back to below set pressure only to rapidly increase again. Rapid cycling reduces capacity and is destructive to the valve seat in addition to subjecting all the moving parts in the valve to excessive wear. E.xcessive back pressure can also cause rapid cycling as discussed above. 

Where outlet pressure losses exceed 10%, bellows valves are often considered. However, substitution of a bellows valve for a conventional valve may not necessarily solve the chatter problem since debits associated with bellows valves reduce the rated capacity of this type valve. Hence, the valve has a tendency to become oversized depending on the amount of back pressure encountered. For this reason, revision of outlet piping to reduce the back pressure within the 10% limit is strongly preferred to the alternative of installing a bellows valve. 

Whenever two-phase flow is encountered in facility piping it is usually in flowlines and interfield transfer lines. Some designers size liquid lines downstream of control valves as two-phase lines. The amount of gas involved in these lines is low and thus the lines are often sized as singlephase liquid lines. Oversizing two-phase lines can lead to increased slugging and thus as small a diameter as possible should be used consistent with pressure drop available and velocity constraints discussed in Volume 1. 

Oversizing regulating valves should be avoided, as this will result in poor regulation characteristics the valve operating in an almost shut position and creating a very high local velocity. 

Once the unit is running well, it is often assumed that the aeration system is sized properly, but changes in the catalyst physical properties and/or catalyst circulation rate may require a different purge rate. It should be noted that aeration rate is directly proportional to catalyst circulation rate. Trends of the E-cat properties can indicate changes in the particle size distribution, which may require changes in the aeration rate. Restriction orifices could be oversized, undersized, or plugged with catalyst, resulting in over-aeration, under-aeration, or no aeration. All these phenomena cause low pressure buildup and low slide valve differential. 

Bare-bulb thermocouples and oversized cooling-water valves arc often used to improve controllability. 

On the surface, this story sounds crazy. But, let s see what happened. This deaerator had been designed for a much smaller flow of 160°F BFW, and a much larger flow of hot-steam condensate, than are current operations. The cold BFW feed line had been oversized, but the steam line was of marginal size. As the demand for hot BFW increased, the cold-BFW level-control valve opened. This reduced the temperature and pressure in the deaerator drum. In response, the steam pressure-control valve also opened. But when the cold-BFW level-control valve was 40 percent open, the steam pressure-control valve was 100 percent. Steam flow was now maxed out. 

The flows required for thermal relief are very small, and there are special thermal relief valves on the market that accommodate this specific application. Oversizing a thermal relief valve is never a good idea, and orifice sizes preferably below API orifice D are recommended. 

In all cases where combinations of bursting disc devices with SRVs are used, measures must be taken to ensure that the space between the valve seat and the bursting disc is kept at atmospheric pressure Any increase of pressure in this cavity due to, for example, temperature changes, minute pressure leaks, and so forth, will result in a dramatic and uncontrolled change in opening pressure of the safety system. Also, with this combination the SRV must be 10% oversized to accommodate the eventual pressure drop over the bursting disc. 

We have seen that it is dangerous to oversize the valve the ability to adjust flow both compensates for this and limits unnecessary product loss (Figure 5.41). 

The liquid capacity is >30% total (volumetric capacity) If a spring valve is required, it should be a valve with liquid trim. Preferably, however, a modulating pilot-operated valve can be used. The modulating pilot ensures stability of operation whatever the phase and cannot be oversized as it adapts its flow to the need of the system. 

Oversizing of the valve (as a rule of thumb chatter is starting at 25% above necessary/calculated capacity) 1. Use smaller size valves. 2. Reduce the lift of the valve (only possible with pilot-operated safety relief valve, POSRV). 

At this section of the plant in the past, chemical process operators (not maintenance mechanics) handled the tank-filling step in the hydrotest process. Operators would typically drape a fire hose into an oversize roof nozzle and fill the tank from the nearest hydrant. In this case, it was reported the two mechanics asked their maintenance supervisor if they could connect the hose to a flanged lower-valved nozzle on the tank. The supervisor remembers requesting the mechanics to roll a blind flange at the top for venting purposes if they used the lower nozzle for filling. There must have been a miscommunication. Obviously the mechanics failed to understand the dynamics of the filling operation when they chose to open only the two 1/2-inch top vents. 

Steam heat provided the fastest recovery time of any system because of the oversize source available in the boiler room. It offered a uniform mold temperature, as do all liquid systems, but is limited to about 350F (177C) maximum. Steam heat is also messy and requires good maintenance, or rusty pipes and leaks become all too common. Steam controls and the accompanying valves are expensive and many are not dependable. 

Do not oversize reducing or regulating valves. Oversizing causes chatter and excessive wear. 

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