Following from my presentation in Valve World Conference 2016 in Dusseldorf about “Top Entry Line Valves Design Consideration”, this article gives a brief summary of what explained in the conference.
Picture 1: Pipeline from offshore platform with top entry valves
The aim of the presentation was to gain general knowledge about pipeline valves in the offshore industry including their general features and some of the design considerations. The question is that why pipeline valves are important? That is because they are largest, heaviest, most expensive and complex valves with the longest delivery time in an offshore plant.
Picture 1 shows the export pipeline from the platform to the shore. The pipeline includes two parts of topside and subsea with two different design codes of ASME and DNV. The export line valves are installed on the pipeline on the platform upstream of the subsea section and they are designed based on ASME code.
PIG refers to Piping Injected Gadget that is run into the pipeline for purposes such as cleaning and maintenance. These valves are designed normally according to API 6D standard. Pipeline valves are top entry with the access to the valve internals from the top. They have one piece body design which gives better mechanical resistance to the pipeline loads in comparison with the split body or side entry design. The valves are welded to the pipeline that reduces the leakage possibility compared to flange connection. The other advantage of top entry valves is that they save two pipeline flanges and two valve flanges since they are welded to the pipeline.
Picture 2: Top entry and side entry ball valves comparison
Picture 2 illustrates comparison between top entry and side entry valves. The main advantage of top entry design for pipeline valves is inline maintenance. It means that there is no need to remove the valve from the pipeline in case of maintenance. In addition, top entry design gives more flexibility in stem enlargement in case of high torque requirements. The welding of the valve to the pipeline is done through pup piece or pup piece plus transition piece. Transition piece can be required due to thickness difference between the valve and pup piece, or welding between two different materials.
Picture 3: Welding of a 30” ball valve
The welding could be more challenging if the valve and the connected pipe have dissimilar materials. Picture 3 illustrates welding of a 30” ball valve. Top entry valves can be Through Conduit Gate Type as an alternative. The selection between the ball and through conduit gate valves depends on parameters such as cost, delivery time, valve dimensions, space availability, weight, client preference, etc. These valves are actuated. Actuator options are hydraulic, pneumatic and electrical. The choice of actuator depends on factors such as valve size and pressure class, required torque to open and close the valve, valve opening and closing time as well as valve failure mode.
Picture 4: 38”Ball Valve with Cylindrical Nut on Body and Bonnet
There are some ways to reduce the weight of top entry valves. The first one is to calculate the body wall thickness based on ASME Section VIII Div.02 instead of ASME B16.34. Thickness reduction reduces the end to end, height and the weight of the top entry ball valves. Thickness, end to end, height and weight reduction of a 30” CL1500 Top entry ball valve through wall thickness calculation according to ASME Sec.VIII is given in the table.
Picture 5: FEA on a top entry ball valve
The other way of weight saving is to use cylindrical nut rather than hexagonal. Using cylindrical nuts save body and bonnet as well as nut area which result in saving the weight. Picture 4 shows the 38”valve class 1500 with electrical actuator and cylindrical nuts.
In addition, the valve shall be designed to withstand design pressure, hydro test pressure and also pipeline loads. Proper design of the pressure containing parts like body, bonnet, ball, seat, pup piece and bolts to withstand the loads are more important. Finite element analysis (Picture 5) is done on the valve parts to make sure that they are strong enough to withstand the loads.