Tim McMahon, Director of Global Product Marketing for Fisher Rotary Valves at Emerson, writes in the September 2019 issue of Mining Engineering that mining operations require special valves to handle slurries. His article is titled Testing Slurry Valves for Reliability and it’s summarized below.
Mining operations create various types of slurries. The mines then transport these slurries through pipelines. Severe service valves must be used in slurry applications and they must be tested to ensure reliability. This article describes how Emerson designs and tests its Fisher valves for use in mining slurry applications.
Slurry Pipeline
Slurry Pipeline Valves
Ball valves are typically used in a slurry pipelines, McMahon explains.
The valves use floating balls that are held between the seats and are not pinned or permanently attached to the shaft that rotates the ball. The inside diameter of the ball typically matches the pipeline inside diameter, enabling the pipeline to be cleaned or inspected with a pig. Pressures can require up to ASME Class 2500 valves and flanges. The valves can get quite large, up to NPS 36 inches.
Slurries are very erosive and can break up a valve quickly if it is not designed and manufactured appropriately, McMahon warns.
High-velocity jets of water with particulate entrained can cut and remove metal from the valve components. A slurry can be abrasive, erosive and — in the case of chemical processes — corrosive. Slurries cause several types of valve problems including clogging, eroding surfaces, accumulation of slurry material, seat failure, failure to shut completely and leakage, among others.
Designing a Slurry Valve
It’s not enough to take a standard valve, add some hardface elements to resist abrasion and call it a slurry valve. Instead, the valve needs to be designed from the ground up for slurry service.
Emerson starts with understanding the application, identifying a list of product attributes that will define the design and then developing technical specifications that must be met through the design cycle. With the technical specification as the criteria, the Emerson design process uses analytical, computational and experimental methods before a product is placed in the market.
A structural analysis ensures the valve body can maintain the structural integrity at the required pressure and meet industry standards, such as ASME B16.34. The valve body is modeled and analyzed using a finite element (FEA) software package, such as ANSYS.
FEA software models and evaluates the structure of the valve and its ability to maintain pressure integrity.
To ensure the analysis model is valid, structural integrity testing is verified via proof-of-design hydro testing. For this test, a valve body is instrumented with strain gauges.
Validating the FEA model with testing using strain gages.
Testing for Reliability
Reliability testing for slurry handling valves is necessary to ensure performance and minimize concerns before a valve design is placed into service. The areas of testing include materials, coatings and seal performance—primarily to address concerns related to erosion and seat leakage.
Emerson’s approach isolates each aspect of what is critical to overall performance to ensure each area is highly reliable. This allows all aspects of a design to be verified under conditions simulating actual service.
A test rig was built to test Emerson’s Z500 slurry ball valve seals under slurry conditions.
This rig tested seals by cycling a slurry through the ball valve
The seat is a critical component of the valve. As the Z500 operates the seat is required to shut off. The two most important aspects of seal performance are resistance to wear and reliable operation. For reliable operation, the valve must consistently move in both directions and maintain the required seat contact with the ball to maintain shutoff as the valve is used to forward and reverse flow or pressure. Ensuring this performance requires testing.
The first step was to cycle test the valve using air. To test this, the downstream or outlet side was pressurized to 1,400 psi providing a 1,400 psi pressure differential. The seat leakage was measured. The valve pressure was then vented.
Next, the upstream or inlet side of the valve pressure was similarly tested. On completion, seat leakage was measured in both directions. Seat leakage in both directions was below API 598 seat leakage criteria.
A slurry test was next. The slurry was made from water and 95 percent silica sand, which tends to be very abrasive. The goal was to introduce an abrasive particulate that would work the seat as it shuttled and possibly pack into any unprotected areas.
The rig was filled so that settled sand was level with the top of valve. A cycle involved the steps similar to the air test. At the completion of more than 1,000 cycles, the seats met API 598 shutoff requirements.
Material testing is also conducted using a scanning electron microscope and energy dispersion spectrometry verified coating thickness, hardness, microstructure and material composition.
Results
Several design iterations were tested, with information from each step improving the design. Upon completion of all the tests and analyses, the Fisher Z500 slurry ball valve was deemed suitable for use with slurries. McMahon says every valve manufacturer should do this type of testing.
Any valve intended for use in mining slurries should be designed and tested for slurry service. A mine customer should ask the valve manufacturer if such testing was done, and, if so, what were the results, and if the valve meets all applicable mining industry regulations and standards.