To start off with the check valve families we use for all our projects. As mentioned before, we divide this family into two branches: bulk and tag items. Bulk check valves are included in Piping Material Class, they shall be suitable for P/T design conditions, material requirements, CA, etc.
However, there are no specific requirements from the Process Department.
Tag items, on the other hand, need to meet specific process requirements that are applicable to specific valves at specific locations in the process plant.
There can be also cases in which dissimilar check valves are used - in series - at one location. One could be a tag item and the other a bulk item.
As mentioned before, tag items are almost exclusively non-slam check valves that are mainly specified due to process requirements.
Typically, this type is situated in the vicinity of a compressor, a downstream pump with high differential pressure, downstream pumps in parallel and in some special cases
Again, also dissimilar check valves - in a configuration of two in series - are considered (partly) tag.
Going back to non-slam check valves, these need to comply with the following process requirements: fast closing, low pressure drop and operate at flow conditions that are indicated in the Process Data Sheet (PDS).
The PDS is the starting point from which we manage the selection and procurement process. After having received the PDS, we devise a Mechanical datasheet which indicates the check valve type, PDS-data, mechanical and material requirements according to relevant Piping Class, the material(s) for the body and internals, corrosion allowance, rating class, etc.
Zooming into the process requirements for non-slam check valves used in gas service, two process parameters stand out: the maximum pressure drop and the various flow rates (minimal, normal, maximum). These parameters can be very relevant given their impact on the plant’s efficiency (and subsequent process costs).
Raw analysis not sufficient
Considering the specific requirements for tag items, the procurement cycle is different from the bulk items. In the last case, we use a five step process (see Figure 1) to go from inquiry to vendor selection.
For tag items, the raw analysis used in the procurement cycle for bulk items, is not sufficient.
So, how can we conduct a raw analysis for non-slam check valves? We could review the mechanical aspect as we can with standard valves: rating, ends, flange facing, material selection, heat treatments, testing, etc.
However, this analysis is not sufficient as we also need to appraise the flow performance of the proposed check valves before to going to the shortlist. This flow performance assessment requires the following data: pressure drop across the check valve for all operating cases, valve pressure drop curves with all operating flows plotted and minimum flow to open fully the check valve, low permanent pressure drop without back-flow and valve chattering, minimum flow on pressure condition to open fully the check valve, crack open pressure. The best way to provide the required information is in the form of the response curve plotted with fluid operating conditions (see Figure 2).
Insights for projects
Project feedback has garnered several insights into the procurement process of non-slam check valves. Based upon the aforementioned flow performance charts, we have had several discussions with suppliers regarding design changes and/or material selection to prevent issues such as chattering during unstable operating zones. Based upon these changes, pressure drops were reviewed in an updated flow performance chart. It is also important to establish between the EPC/end user and the supplier whether certain process parameters are incidental or more frequent. Based upon this assessment, certain parameters, such as minimal flow, could be incorporated into the design/material selection.
In general, the communication between supplier and EPC is crucial in selecting – and sizing – the right non-slam check valve for the job.