Orifice hole pressure drop

The orifice hole pressure drop. This is the liquid head, which has to be converted to velocity as the liquid flows through the orifice holes on the tray deck. 

Vg = velocity of vapor passing up through the chimneys, ft/s The orifice hole pressure drop is... 

The dry hole pressure drop for a sieve tray may be calculated by use of the following equation for thick plate orifices... 

Sieve Plates. The conventional sieve or perforated plate is inexpensive and the simplest of the devices normally used. The contacting orifices in the conventional sieve plate are holes that measure 1 to 12 mm diameter and exhibit ratios of open area to active area ranging from 1 20 to 1 7. If the open area is too small, the pressure drop across the plate is excessive if the open area is too large, the Hquid weeps or dumps through the holes. 

Orifice plate A metal plate with a hole of diameter smaller than the pipe or duct run in which it is fitted. The pressure drop that takes place across the plate is used to calculate the fluid velocity. 

Orifice coefficient. Figure 8-129, read at 0.41 tray/hole gives Cq orifice coefficient = 0.75 Hole velocity = 347/8.66 = 40.06 fps Dry Tray pressure drop... 

The pressure drop over the plates is an important design consideration. There are two main sources of pressure loss that due to vapour flow through the holes (an orifice loss), and that due to the static head of liquid on the plate. 

The simplest and most common device for measuring flow rate in a pipe is the orifice meter, illustrated in Fig. 10-7. This is an obstruction meter that consists of a plate with a hole in it that is inserted into the pipe, and the pressure drop across the plate is measured. The major difference between this device and the venturi and nozzle meters is the fact that the fluid stream leaving the orifice hole contracts to an area considerably smaller than that of the orifice hole itself. This is called the vena contracta, and it occurs because the fluid has considerable inward radial momentum as it converges into the orifice hole, which causes it to continue to flow inward for a distance downstream of the orifice before it starts to expand to fill the pipe. If the pipe diameter is D, the orifice diameter is d, and the diameter of the vena contracta is d2, the contraction ratio for the vena contracta is defined as Cc = A2/A0 = (d2/d)2. For highly turbulent flow, Cc 0.6. 

An orifice meter with a hole of 1 in. diameter is inserted into a l- in. sch 40 line carrying SAE 10 lube oil at 70°F (SG = 0.93). A manometer using water as the manometer fluid is used to measure the orifice pressure drop and reads 8 in. What is the flow rate of the oil, in gpm ... 

A 2 in. sch 40 pipe is carrying water at a flow rate of 8 gpm. The flow rate is measured by means of an orifice with a 1.6 in. diameter hole. The pressure drop across the orifice is measured using a manometer containing an oil of SG 1.3. 

When vapor flows through a tray deck, the vapor velocity increases as the vapor flows through the small openings provided by the valve caps, or sieve holes. The energy to increase the vapor velocity comes from the pressure of the flowing vapor. A common example of this is the pressure drop we measure across an orifice plate. If we have a pipeline velocity of 2 ft/s and an orifice plate hole velocity of 40 ft/s, then the energy needed to accelerate the vapor as it flows through the orifice plate comes from the pressure drop of the vapor itself. 

The standard method of measuring flows in a process plant is by use of the orifice plate and orifice flanges, shown in Fig. 6.7. Actually, we rarely measure flows directly. More commonly, we measure the pressure drop across an orifice plate. This pressure drop is due to the increase in kinetic energy of a fluid as it accelerates through the small hole in the orifice plate. The energy to provide the increased velocity comes from the pressure of the flowing fluid, in accordance with the following ... 

One way out of this problem, is to increase APL, the pressure drop of the liquid flowing through the orifice holes. This could be done, by increasing the orifice hole liquid velocity. We could drill fewer orifice holes. Unfortunately, this would decrease the number of drip points per square foot of tower area (6 to 10 is a good target). This would reduce vapor-liquid contacting efficiency. Or, we could have smaller orifice holes. But too small a hole would probably plug with corrosion products. 

Low dry tray pressure drop. On sieve and fixed valve trays, this means high (>11 percent) fractional hole area. On moving valve trays, this means venturi valves (smooth orifices) or long-legged valves (>15 percent slot area). On all trays, the channeling tendency and severity escalate rapidly as the dry pressure drop diminishes (e.g., as fractional hole area increases). 

The velocity based on the hole area is v . The pressure Pi is the pressure upstream of the orifice, typically about 1 pipe diameter upstream, the pressure P2 is the pressure at the vena contracta, where the flow passes through a minimum area which is less than the orifice area, and the pressure P3 is the pressure downstream of the vena contracta after pressure recovery associated with deceleration of the fluid. The velocity of approach factor 1 — (AJA)2 accounts for the kinetic energy approaching the orifice, and the orifice coefficient or discharge coefficient C accounts for the vena contracta. The location of the vena contracta varies with AJA, but is about 0.7 pipe diameter for AJA < 0.25. The factor 1 — AJA accounts for pressure recovery. Pressure recovery is complete by about 4 to 8 pipe diameters downstream of the orifice. The permanent pressure drop is Pi — P3. When the orifice is at the end of pipe, discharging directly into a large chamber, there is negligible pressure recovery, the permanent pressure drop is Pi — P2, and the last equality in Eq. (6-111) does not apply. Instead, P2 = P3. Equation (6-111) may also be used for flow across a perforated plate with open area A and total area A. The location of the vena contracta and complete recovery would scale not with the vessel or pipe diameter in which the plate is installed, but with the hole diameter and pitch between holes. 

Klein s method requires that in the diy pressure drop equation, Eq. (6.42], the vapor hole velocity uh is based on the area of holes on the deck of a valve tray (Arfo), and not on the slot area. In many valve trays, the orifice diameter is standardized at ll%2 in. Values of Kc and K0 are listed in Table 6.9. K0 is listed as a function of the tray deck thickness t, not the valve thickness. When the tray deck thickness it not listed in Table 6.9, K0 can be evaluated from... 

Uo = average hole velocity, cm/s Udow = average velocity in downcomer, cm/s Re = orifice Reynolds number, ( = doUopd/fXd) APo = orifice pressure drop, g/cm-s ... 

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