CHECK VALVES IN ETHYLENE PLANTS

Transcription

1 CHECK VALVES IN ETHYLENE PLANTS Gerard Schreurs R&D engineer Mokveld Valves 6. V. Gouda Holland April 26,2006 Prepared for the AIChE Spring National Meeting, EPC Conference Orlando, Florida April 23-27,2006

2 CHECK VALVES IN ETHYLENE PLANTS Gerard Schreurs R&D engineer Mokveld Valves 6. V. Gouda Holland Abstract Check valves are important devices in any plant. They protect millions of dollars worth of equipment and are therefore to be regarded as critical. Careful selection and sizing is paramount in order to ensure a trouble free operation. This paper describes the most common check valves available on the market, and their advantages and drawbacks. Furthermore guidelines are given on a proper selection, and finding a balance between cost (including energy loss due to pressure drop) and performance (speed of closure and reliability).

3 Introduction From a quick look, all check valves may appear to be similar. On closer examination, it becomes apparent that there are important differences in the way that check valves operate. First of all, we have to ask ourselves why we are using check valves, and what we expect from a check valve. Combined with an understanding of function and design this will enable a better comparison of check valves. Also, this will enable us to formulate the demands that should be imposed on the ideal check valve for a certain application. It is important to realise that a check valve will always be a compromise between pressure drop and speed of closure, and that the best check valve for your application is tailor-made with respect to size, spring and material selection. In the early days, check valves were regarded as passive devices that had to do a simple job: prevent the flow from reversing. One of the earliest types is the swing type check valve. Only much later, users started to realise the importance of lower pressure drops, and axisymmetrical designs with optimised flow paths were introduced like the Mokveld TKZ check valve. Over the last decade, with the aid of CFD computer programs, a new step was made towards lower pressure drops and improved performance. Nowadays, check valves are available with a variety of options like stronger springs, adjusted body sizes, materials and design features to allow tight shut-off even under high-temperature or cryogenic service conditions.

4 Demands On Check Valves There is a number of demands that can be imposed on check valves, some of which may seem conflicting: 1. Prevent backflow All check valves prevent backflow. There is however a big difference in the amount of backflow (reverse velocity) that different check valve designs allow. 2. Minimise reverse velocity at moment of closing In order to prevent damage to the equipment that is to be protected by the check valve, a check valve should be able to quickly follow the reversing flow, and, ideally, close at the moment of zero flow. Valves with larger disc travels will be slower in responding to the reversing flow. 3. Prevent slam at moment of closing A check valve that delays closing, may induce water hammer or slamming once the disc hits the seat. The pressure surge associated with the water hammer may exceed the design pressure of the equipment. A well-designed check valve will follow the diminishing flow directly, and be (almost) closed at the moment of flow reversal. Ideally, the closing force (spring) can be tailored to the flow conditions. 4. Fully open under working conditions To ensure a stable, non-oscillating performance, a check valve should always be fully opened under design flow conditions. This means that the valve should be fully stroked against a backstop. 5. Bubble-tight shut-off Bubble-tight shut-off can not only be achieved with sor seals, but also with carefully machined metal-to-metal discs and seats. This allows operation of the check valve under extended temperature ranges. 6. Last but not least: low pressure drop One thing that is often forgotten when selecting a check valve, is the fact that a valve with a high pressure drop will cost money during its entire life cycle. The total cost of ownership of a check valve with a low pressure drop may easily justify the higher price.

5 Different Types Of Cheekvalves a) Swing type b, b) Butterfly C) Tilting disc d) Split disc el Piston type f) Plug type g) Ring type h) Membrane Figure 1 Examples of check valve designs The great variety of check valves available on the market Is demonstrated by the examples in Figure 1 above. The most important types are the swing check valve, the dual-disc (or split disc) check valve, the tilting disc check valve and axial flow type check valves, and they all have different specific applications. Check valves In ethykne plants AIChE Spring National Meeting 2006 Gerard Schnurs, Mokveld Valves B.V.

6 Swing Check Valves Figure 2 The shut-off of a swing check valve depends on gravity and on reverse flow. During normal operation, the disc balances on the flow. The hinge of the disc is located outside the flow path. Upon flow reversal, the disc has to travel through a 90-degree arc. This makes the check valve slow in responding to reversing flow, increasing the risk of slamming. The pressure drop will be relatively high. Dual Disc Type Check VaEves The dual disc type check valve is an improvement on the swing check valve. By splitting the disc in two, the mass of the disc is reduced, as well as the travel required to close. Check valves in cthykne plants AIChE Spring National Meeting 2006 Gerard Schnurs, Mokveld Valves B.V.

7 This type will close quicker than the swing check, but the pressure drop will still be high because of the two balancing doors in the flow path. Wafer type check valves are often of the dual disc type. Although this type may be less expensive, wafer type valves are not recommended for hydrocarbon service. In case of fire, the flange studs may expand, causing leakage. Typically, they are not used in cryogenic and high temperature services and pressure classes greater than ASME Class 600. Tilting Disc Check Valves Figure 4 This type of check valve is another improvement of the swing check valve. The flow area is enlarged, and the hinge pin located in the flow path. This check valve will close quicker than a swing check valve, because it has to travel through a smaller arc before closing. The disc is, however, still balancing on the flow. This type valve is available with external hydraulic dampening to reduce slamming. Axial Flow Qpe Check Valves Check valves In ethytene pki* AIChE Spring National Meeting 2006 Gerard Schnurs, Mokveld Valves 6.V.

8 Figure 5 There are two types of axial flow check valves: the centrally guided piston type and the ring type check valve. What these valves have in common is a smooth flow path that gradually accelerates and decelerates the flow in order to minimize the pressure drop. The centrally guided piston design allows the use of strong springs, which is beneficial for the closing speed and reduces reverse flow and water hammer to a minimum. The rotational symmetry of the disc and seat enable a positive shutoff, even with metal-to-metal seating. This type of valve is available in high pressure ratings up to API , and in sizes of 48" and above for class 600 pressure ratings. Special Applications In Ethylene Plants There are several services in an ethylene plant that require specialized types of check valves: Streams on the discharge of large pumps such as those for Quench Oil and Quench Water circulation require a quick-closing, non-slam check valve. This prevents backflow from reversing the pump rotation and the non-slam feature alleviates liquid hammer. Streams on the discharge of Charge Gas and Ethylene Refrigerant Compressors use a low pressure drop, non-slam check valve at the discharge. Streams on the discharge of Propylene Refrigerant Compressors use low oressure droo. non-slam check valves at each suction line and at the discharge. valves by manufacturers with extensive experience with compressor lines are typically used. LI Super high-pressure streams (1500 psig) on the discharge of each cracking heater steam drum use stop-check non-return valves. A stopcheck valve is a special combination of a check valve and an on+ff valve, where the disc of the check valve can be actuated. These valves prevent backflow and act as a second block valve for isolation. In some cases, special stop-check valves are also used in the transfer lines from each cracking heater before they enter the main collection header. They

9 prevent backflow into the firebox in case of a tube rupture and also serve as a second block valve for isolation. Special Considerations In Ethylene Plants It is important to consider other components in the piping system when selecting a location for the check valve. For example, the discharge check valve for a Cmcked Gas Compressor should be located downstream of the recycle piping take-off point in the compressor discharge piping. In refrigemtion systems where compressors are motor driven, check valves are sometimes used to hold full pressure ( psi) downstream of the check valve compared to nearly atmosphere pressure on the inlet as the compressor's discharge is evacuated prior to startup. In this case the check valve has to provide a tight seal. The alternative may be to buy large MOV valves, so the check valve provides a low cost alternative if it is reliable. o Just a few years ago, there was a major fire in an ethylene plant on the US Gulf Coast when a part of a check valve came apart in the process gas compression system. The result was an explosion and plant shutdown for several weeks. Reliability and mechanical integrity is of vital importance when selecting check valves for light hydrocarbon service. For a typical ethylene refrigeration system with compressor size around 10,000 KW, adding an extra 1 psi to the discharge would increase power requirements by around 35 KW. On an annual basis this could increase the opemting costs by $20,000 (depending on the cost of energy).

10 Dampened Versus Non-Dampened Some designs employ an external dampening device to avoid slamming. The dtawback of delayed closing however, is that backflow is allowed for a longer period of time, increasing the risk for the surrounding equipment. Figure 2 below illustrates this effect. During the extra time needed to close the valve in a controlled manner, the backflow velocity builds up, making the cure worse than the symptom. Furthermore it is difficult to compare the performance of dampened check valves. dampened check valve fast-acting non-slam check nlve closed To I, e? 3 0 2?; $ : c 4 -" 3, C -.! < Figure 6 Effects of external dampening on check valve performance Checkvalves in ethylene plants AIChE Spring National Meeting 2006 Gerard Schreurs, Mokveld Valves B.V.

11 Principle Of Operation Of Axial Flow Check Valve Figure 7 Axial flow check valve The axial flow check valve will be fully opened under normal flow conditions. The flow path is smooth, and the relatively big frontal area of the disc enables the use of a strong spring. The strong spring assists in fast closing at the moment that the flow tends to reverse. The strength of the sprlng does not change the overall pressure drop over the valve, since it is fully open! Pressure drop curve Figure 8 Typical prwure drop curve Check valves in cthykne plants AIChE Spring Nntional Meeting 2006 Gerard Schnurs, Mok~eld Valves B.V.

12 Figute 8 above shows a typical pressure drop curve for a check valve. The pressure drop is proportional to the flow squared once the valve is fully open. Dynamic Behavior A lot of investigation has been done into the dynamic behavior (closing behavior) of check valves. In 1986, the Delft Hydraulic Laboratory, together with Mokveld Valves B.V. published a method for comparing the dynamic behavior of different types of check valves1. This method, the "dimensionless dynamic characteristic" has now been adopted by manufacturers and end users worldwide to cornpart the performance of check valves in this respect. Based on the flow deceleration at the moment of closing as an input, the method predicts the backflow velocity for a given check valve as a function of the installed spring foxe. The corresponding pressure rise may than be calculated using Joukowski's formula: with : = Jou kowski pressure rise (Pa) = specific weight of fluid (kdm3) = propagation speed of pressure waves (m/s) = stopped back flow velocity (m/s) Check valves In anylene ptam AIChE Spring National Meeting 2006 Gerard Schnurs, Mokveld Valves B.V. Figure 9. 48" Check Valves ready for transport

13 Summary When selecting a check valve it is important to be aware of the interaction with the system. Aspects to be considered are the dynamic performance (closing time) and static performance (pressure drop). A check valve which allows substantial backflow or slams when closing may harm the equipment to be protected or the surrounding piping. A check valve with a low pressure drop will save energy throughout its entire operational life. The ideal check valve for any application is carefully selected and engineered with respect to both aspects. Gouda, Holland March 2006 References: 1. H. KOFKZIER, Mokveld Valves; A.C.H. KRUISBRINK and C.S.W. LAVOOU, Delft Hydraulic Laboratory: "Dynamic Behaviour Of Large Non-Return Valves" sth International Con&rence on Pressure Surges, Hannover September 1986