mar 03, 2023

Slurry pipe sanding: BBA approach for load case design criteria in brownfield projects

This blog article is intended to explain verification methods for slurry piping systems that are subject to periodic sanding. These methods ensure proper engineering for operational safety without overdesigning.

While designing slurry piping systems, establishing one specific design criteria may come to the forefront: how to adequately evaluate weight increase in a sanded pipe and which average slurry specific gravity (SG) to use.

BBA recently came across this situation on one of its brownfield projects. This is presented as a case study herein.

Case study description
The operator of an existing mineral processing concentrator expressed their concerns with regard to the mechanical stability of a slurry piping for a grinding circuit to prevent supports and joints failures. The Operations team observed recurrent sanding. In such a case, pipes carry extra weight, and BBA was asked to assess the average slurry SG in a sanded pipe to be used in a pipe and support design review.

Transported solids (ground ore) have an SG equal to 3.6. Initially, it was suggested to assume an average slurry SG during sanding equal to that of an SG for solids with stationary deposits occupying the full pipe cross-section.

BBA dissected the relevant operations and proposed using another value. The main determining factor of said value was the notion that the stationary bed of solid particles would have voids, whereas the voids were filled with a carrier liquid. Voidage could be estimated either through lab tests or, with some approximation, analytically.

For proper estimation, the maximum settled bed volume fraction (C_max) first needed to be established and then used to derive the corresponding overall specific gravity of the stationary bed.

Another factor we looked for that would affect average SG was the area of pipe cross-section that could be filled with deposited material:
- when flow continues.
- when pipeline drainage is completed.
Sanding mechanics

Sanding happens in a settling slurry flow in pressurized pipes when line velocity falls below deposition velocity value (V_D) while staying above the velocity at or above which the bed in the lower half of the pipe is stationary. For more information, please refer to pages 198–199 and Figure 7-5 in Slurry Systems Handbook, Second edition, B. Abulnaga, McGraw Hill, 2021.

Continuing operations with velocity below V_D would lead to stationary bed buildup, until the pump stops, and the line is drained thereafter. At the end of the drainage cycle, due to the pipe configuration and the number and location of drains, operators may observe even heavier sanding in the specific sections of the pipeline.

Solids in a stationary bed may be deposited to a maximum settled bed volume fraction, C_max that would result in a (maximum) specific gravity of the settled bed calculated with respect to solids SG as follows.

SG of stationary bed = SG_s x C_max +(1-C_max) x SG_l,

where SG_s – solids SG (=3.6 in the subject circuit)

SG_l – carrier liquid SG, water with approximate SG=1.0

Please refer to Figure 1 and the footnote for C_max definition (courtesy of SRC Pipe Flow Technology Center).
$BBA Blog Slurry pipe sanding Figure1 Maximumsettlebedvolumefraction EN$
Figure 1: Cmax definition
C_max is a function of particle shape (the more angular the lower the value, the more round, the higher the value) and particle size distribution (not absolute particle size, the broader the distribution, the higher the C_max as the smaller particles in the distribution can fit in the void spaces between the larger particles resulting in a tighter packing).
Case study results

Please refer to the isometric sketch of the piping (Fig. 2) for the configuration that has been analyzed.

Based on the above description and after having reviewed the piping arrangement, BBA assumed that there were likely two types of material accumulation with respect to the operating mode and piping section location when sanding takes place.

Case 1: Flow continues. Pipe might be filled up to 35% of pipe height (diameter). This criterion seems reasonable, since the heavier deposition would trip the pump as the discharge pressure would rise above the control loop set point.

Case 2: Drainage initiated. Material tends to flow and roll down to the lower horizontal section, further increasing the plugging therein. At this point, water flush or mechanical purge of material may be required to clean the pipe in preparation for the next operation sequence. The lower section was assumed to be occupied by stationary deposit, up to 65% of pipe height (diameter).

Following the method described in technical literature, calculations of C_max were based on the PSD curve (not presented herein as a reference) and yielded the maximum bed volume fraction C_max equal to 0.71. Such high compaction seems to be in line with the notion of higher C_max values for a broad PSD. The resulting SG of the stationary deposit was derived as equal to 2.85.
Figure 2: Isometric sketch
Simplified schematic of sanded pipe cross‑section with a stationary deposit is illustrated in Figure 3 below.
Figure 3: Cross-section of partially sanded pipe
Please refer to Table 1 below for the summary of calculation results for the average slurry SG in the reviewed pipe under sanding conditions.
Table 1: Linear weight loads in sanded slurry pipe, ND=16”, Std. SCH, ¼” rubber lining
Thus, average SG of slurry in Case 1 is 2.15. BBA proposed using SG=2.15 for all sections of the system undergoing sanding, except the lower horizontal one.

Average slurry SG in Case 2 is 2.55. BBA proposed using SG=2.55 for the lower horizontal section undergoing sanding.

Another observation was that weight load increase in said pipe will be in the range of 12% to 30%. Such an increase unlikely imposes any risk to the originally designed piping system.
Conclusions
BBA believes that average SG derived in the range of 2.15 and 2.55 will provide for reliable input into pipe stress analysis calculations. These values are substantially lower than the SG=3.6 initially suggested. And, while providing for operational safety, using this SG range would help avoid overdesign.

When performing pipe stress calculation, designers should be careful when selecting design criteria for the average slurry specific gravity under sanding conditions. The following are the factors that need to be accounted for while establishing said parameter:
1. Maximum bed volume fraction (C_max) derived by laboratory test work or analytically
2. Specific gravity of solids
3. Particle size distribution (PSD), if analytical calculations of C_max need to be performed
4. Cross-section area of the pipe assumed to be filled with stationary deposit
To disclose the root cause of recurring sanding, an investigation for specific application should be conducted prior to reinforcing piping. Under some circumstances, redesigning the pumping system may be advised to avoid sanding recurrence.

BBA applies a different approach that addresses slurry piping sanding while designing a brand-new system in a greenfield project. However, this is a topic for a separate blog article.