Practical Process Engineering

Pilot operated safety valve withe remote sensing option

Safety valves may have some types of flow instabilities. One of them is called "Chattering". During chattering, valve opens and closes rapidly and disc touches the seat several times per second. This causes premature failuire of valve internals. Tow major root causes are: 1- Oversized valve: When rated flow (actual capacity) is much greater than required flow. As a rule of thumb, required flow should not less than 25% of the rated flow or the actual orifice size should not greater than 3~5 times of the effective orifice size. Here I used four new terms and will express them in details later today. 2- Exessive pressure drop in inlet pipe: Pressure drop in inlet pipe should be limited to 3% of the set pressure. There is always a pressure difference between vessel static pressure and static pressure right under the disk during the reiliving. When the valve is closed, these pressures are the same, but after the flow is established, they differ. Exessive pressure drop causes static pressure under the seat drops below the set pressure suddenly after the valve opens. The valve closes. Static pressure grows and valve opens again. This makes a high fequency cycle and flow instability. API limits the pressure drop to 3%. (low frequency opening/closing is called cycling and if disk moves up and down but does not touch the seat, this phenomenon is called fluttering) Pilot operated safet valves are good options to care this type of instablity. If you can't reduce the pressure drop due to any reason, use a pilot-operated safety valve with remote sensing. In this way, pilot senses the vessel static pressure directly and it controls main valve operation based on this pressure not the actual pressure under the disc.

These three pictures depict one of the main advantages of pilot-operated safety valves. While system pressure is increasing, the net force exerted on the piston/diaphragm increases. So, closing force increases and we have better tightness and less leakage. This is why pilot operated safety valves are the best options when maximum operating pressure is close to set pressure. Please review previous posts about design pressure. As a rule of thumb, maximum normal operating pressure is 90% of the design pressure (=set pressure= MAWP in most applications). But, in high pressure applications, we add only a few pounds or bars over the maximum normal operating pressure and ignore this 90% rule. So, the these two pressures are close in high pressure applications. Spring-loaded safety valves may leak and start simmering before system pressure meets the exact set pressure.

Modulating-action opens gradually with some overpressure

Pop-action opens fully with no overpressure

Pilot operated safety valve: The 3rd type of safety valve widely used in process industries is pilot-operated type. Main valve comes with a small pilot valve that controls opening and closing of the main valve. Pilot has a similar structure as spring-loaded safety valves and the main valve have piston or diaphragm. Pilot connects the top side of the piston/diaphragm to the bottom side so that they are under the same pressure. But the type side is a little bit wider. Therefore, net force (force acts on the top side - force acts on the bottom side) keeps the valve tightly closed. Once the pressure meets the set pressure, pilot valve pops and realeases the top side pressure to atmosphere or discharge side of the safety. Piston/diaphragm moves upward and main valve opens. There are two type of pilots: 1- Pop-action: It causes main valve opens fully at set pressure and there is no overpressure. Pressure is realeased suddenly and such sudden opening and closing put more more force upon the main valve seat. It is not recommended for liquid services. 2- Modulating action: It opens gradually to realese only some enough fluid and keep the pressure vessel safe.

You may ask why is it important to recognize critical flow inside a safety valve? Please review the tables I posted earlier for critical pressure ratio. All of them are above 0.5 (50%). As an estimation, if the pressure ratio is greater than critical ratio, there is no critical flow and if the pressure ratio across the vena contracta drops below this critical ratio, the flow is choked= critical flow. Critical pressure ratio for a safety valve is nothing more than the ratio of backpressure to relieving pressure (all in absolute value). Exessive pressure drop (low back pressure) causes critical flow inside a safety valve. In brief, fluid flow inside a conventional and balanced safety valve are critical. Sizing of safety valves depend directly upon the flow regime. So bp is very important because low bp causes the flow across the conventional and balanced safety valves be always choked.

Please take a look at this excerpt from an Aspen HYSYS file. To compute critical pressure ratio, use the ideal gas Cp/Cv not Cp/Cv that belongs to real gas.

API tabulated the critical pressure ratio (Pcf/P1) for some gases using ideal gas K. You can extract ideal gas K from process simulators. Also, API allows to calculate K for mixtures additively when there is no other reliable source is available: K for mixture= Sigma(Xi * Ki); X: Mole fraction

Please note that P1 and Pcf are both absolute pressures and K is ratio of specific heats for an ideal gas. It means you must assume the single component or multicomponent fluid in your case is ideal not real. Make sure you use the right K of ideal gas when extracting data from process simulators.

Let's back to safety valve world. Safety valves have an orifice and it is important to us when this orifice choked. Can we predict the excessive pressure drop required for choked flow? API-520 allows us to use a simple formula developed from ideal gas law as a good approximation to predict critical flow as API-520 assumes the flow is isentropic. Let's discover this simple formula:

Before introducing the 3rd type of safety valves, I would like to open a short discussion on critical flow. What is critical flow? We all are familiar with Mach number. It is the ratio of fluid velocity to sound velocity in the medium. Mach number is used to define flow regimes for gas/vapor flow: M<1: Subsonic M=1: Sonic M>1: Supersonic Gas flowing adiabatically in a pipe always expeience an increase velocity. Also, if you like to pass this gas through a vena contracta (Valve, orifice, etc) to reduce the pressure, fluid velocity increases. If velocity reaches sound speed, subsonic flow changes to sonic flow and the volumetric flow rate gets fixed and independent of the pressure drop (delta P) around the vena contracta. The flow is choked and this condition is called critical flow. Here we assume the area of this vena contracta is fixed like the orifice area of a safety valve. However, If volumetric flow passing through a control valve at a certain opening, say 65%, choks, nor the increase of upstream pressure neither the decrease of downstream pressure can change this flow. You have to open that valve to change the valve CV and allow more gas passes the valve. We talk about control valve Cv in future. Please note that if this control valve experiences choked flow due to excessive pressure drop at 100% opening, you have no chance to increase the volumetric flow unless you changed the valve and installed a bigger one. An important note: While the volumetric flow of a gas is constant at choked flow (critical flow), mass flow rate increases easilly by higher upstream pressure as the gas density goes up by a higher pressure. How liquid flow gets choked? Exessive pressure drop across a vena contracta may cause liquid static pressure drops below the vapor pressure at flowing temperature and some of the liquid vaporizes. Vapor has less density compare with liquid and occupy higher volume. So, the liquid expands and needs higher surface area to flow freely, though the area downstream of the vena contracta is fixed. An excessive bp appears and blocks the liquid flow partially. Liquid flow gets fixed and choked. Here there is no difference between volumetric flow rate and mass flow rate and both are choked. Changing the flowing area (for example by opening a semi-open valve) may be a solution. Now you are familiar with the critical flow or choked flow concept. What is the root cause of critical flow? Excessive pressure drop across a vena contracta that leads to higher velocity compareable to sound velocity in the medium.

This graph is only valid for balanced safety valves on liquid services.

This graph is only valid for balanced safty valves on gas/vapor services with set pressure >= 50 psig. For other set pressures, consult vendor. For overpressure= 21%, this factor is 1.

API limits application of balanced safety valves per following rule: Total bp <= 50% of set pressure It means if set pressure= MAWP= 100 psig, total bp for a balanced safety valve must not exceed 50 psi. So, conventional safety valves are limited by their built-up bp and balanced ones are limited by total bp. If due to any reason, some parts of the disc are not fully covered by bellows or piston, higher bp than 50% exerts a closing force on those unbalanced areas leading to less capacity. Therefore, manufacturers provide us with a capacity correction factor so called "backpressure correction factor". API-520 suggests to use following graphs to locate bp correction factor for given bp and overpressure (only 10% and 16%) of balanced safety valves. This graphs are only useable for preliminary safety valve sizing and the designer must contact the vendor for accurate amount of this factor.

Balanced piston vs balanced bellows safety valve