Multiple valves

For wide ranges of operation, multiple valves help keep the efficicncic high (see Figure 7-15). As power output is increased, the lower curve flow... 

For positive displacement pumps, a bypass-type control valve should be furnished to set the primary lube system pressure. The valve should be able to maintain system pressure during pump startup and pump transfers, which includes relieving the capacity of one pump, while both are running. The valve should provide stable, constant pressure during these transients. Flow turndown of 8 to 1 is not unusual. Multiple valves in parallel should be used if a single valve is not suitable. The valve should be sized to operate between 10 and 90% of the flow coefficient (Cv). Additional pressure control valves should be furnished as required to pro ide any of the intermediate pressure levels. 

As the vessel pressure continues to rise above set pressure, the spring is further compressed until the disc is at full lift. The valve is designed to pass its rated capacity at the maximum allowable accumulation (10% for contingencies other than fire, 16% if multiple valves are used or 21 % for fire exposure). 

The code stipulates that when multiple valves are used, only one of them needs to be set at the maximum allowable working pressure (MAWP). The additional valves can be set at up to 105% of the MAWP. For design purposes the maximum allowable working pressure is assumed to be the same as the design pressure. 

Credit for additional height of the flame center for multiple valve installations may be taken by clustering the safety valve discharge pipes to the atmosphere. The following procedure should be used for determining equivalent diameter and exit velocity to be used in the flame center calculation. Diameter and velocity are based on the total acmal area of the clustered vents. 

Maximum Allowable Accumulated Pressure for Multiple-valve Installation (Other Than Fire Exposure)... 

FOR MULTIPLE-VALVE INSTALLATION (OTHER THAN FIRE EXPOSURE)... 

Pipe Orifice Area of a Single Safety Relief in Vapor Service/ Should Not Exceed 2X of the Cross Sectional Area of the Protected Line. 0 Multiple Valves with Staggered Settings May be Required. 

In the case of multiple valves protecting the equipment, at least one valve must always be set at no more than the MAWP. Of course, it is always allowed to set the safety valve at a set pressure lower than MAWP. 

Multiple valves are used for a wide variety of reasons but most common are ... 

In the case of multiple valves, only one valve needs to be set at no more than MAP. The others can be set up to MAP +5% (inclusive). In any case, the required capacity must always be relieved at no more than MAP +10%. As taken from the harmonized standard EN 764-7 paragraphs ... 

While the ASME has two different sections, Section I for fired vessels and Section VIII for unfired vessels, the PED encompass all these conventional vessels (excluding nuclear applications). Therefore, according to PED, unlike with ASME, these above considerations on accumulation and multiple valves installation also apply to boilers and other fired vessels as well as for unfired vessels as long as the pressures are higher than 0.5 barg. 

Note that in the multiple valves case, where one valve is set at PS and the others set staggered above but not more than 5% above PS, it is not said that the valves must be certified at 5% or less overpressure. It is only said that one needs to be sure that the increase pressure will not exceed 10%. 

With these valves, the set pressures of multiple valves can be staggered up to 5% accumulation and still can respect the 10% maximum accumulation for full capacity. 

For multiple valves cases, in principle, each application would need to be reviewed prior to any commitment. It is important to note here that the flow coefficient to be used may be different than the usual ones published in manufacturers catalogues. 

C. ASME VIII - G-125 Accumulation of the pressure vessel above MAWP for multiple valves is 16% or 0.27 barg (Figure 4.3). 

D. ASME VIII - lIG-125 Relationship between set pressure, overpressure, accumulation and MAWP on multiple valves (Figure 4.4). 

What the PED does say in its ESR related to SRVs is that pressure-limit- PED categories ing devices must prevent the maximum allowable pressure being permanently exceeded. The safety valve (SV) should be set at PS (= MAP) or lower, with exception of momentary pressure surge (accumulation) which is 10% of PS in all cases (multiple valves also) and except for fire, this can be higher if proven safe. This is much more general but definitely differs with ASME as described in detail in Section 3.6. 

The design almost always exceeds the limits of API 526 but is, of course, fully conforming to the ASME VIII code and can be stamped per the code. Since they can handle extremely high temperatures and pressures, one valve can handle many times the capacity of multiple valves. 

In special cases such as multiple valve installation, the aerodynamic noise level may warrant more detailed determination. However, this is a decision of the system designer familiar with all the local conditions and regulations and, more importantly, implantation of the valves within the site. 

Constant pressure variations in inlet or/and outlet piping 1. Rearrange piping. 2. Use balanced bellow valves if pressure variations occur in outlet piping. 3. Use POSRV. 4. Use multiple valves with different sizes in stagger set. 

Maximum allowable accumulated pressure for multiple valve installation (other than fire exposure)... 

Multiple valves have been constructed on a PDMS valve control layer (see Figure 3.27) [167]. Multilayer soft lithography has been used to generate the valve control layer (4 mm thick for strength) plus a fluid layer (40 pm thick) on PDMS. The small Young modulus ( 750 kPa) of PDMS allows a large deflection (1 mm) to be produced using a small actuation force ( lOOkPa on a 100- x 100-pm valve area). The response time is on the order of 1 ms. Round channels were... 

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