Volume SH 5: Part
1 Valves for Slurry Applications
This part surveys the various
generic valve types used in slurry service and discusses
the various factors which affect the selection of the
valve most appropriate to the application in question.
For each valve type, information on design, size and
duty and advantages/limitations will be given. Factors
affecting valve performance such as valve pressure-temperature
rating, valve flow path configuration, valve operation
/ control, valve reliability and system layout are discussed.
Guidelines/criteria for slurry and bulk solids valve
selection are also briefly outlined.
Volume SH 5: Part
2 Selection of Slurry Valves
This part
considers the selection of valves (isolation, change-over,
regulating, mixing, non-return, control and safety)
for slurry applications.
Many standard
valve designs are often found to be unsuitable for slurry
applications. However, some standard valve designs can
be utilised successfully in installations for non-abrasive
slurry. Valves for abrasive slurry must be selected
carefully and a detailed knowledge of the slurry properties
is essential. There is no safe limitation of solids
concentration which will produce a non-abrasive slurry
using hard, angular solids.
Valve manufacturers
adopt different design principles for their products
and similar valves from different manufacturers can
be significantly different. The diameter of the valve
flow port may be considerably smaller than the nominal
pipe size suggests. Valve selection can only be successful
when accurate information is available. The selection
of valves based on engineering handbooks may produce
unpredictable hydraulic and wear characteristics.
The issue
of whether a slurry can be considered non-abrasive or
abrasive is dependent upon the properties of the materials
in direct flowing-contact. Standard valves are manufactured
from relatively soft materials although some valves
dedicated to special applications, such as oil-field
installations, are manufactured from harder materials.
Many standard valves can be modified by using coatings
or linings to improve abrasion/erosion resistance. The
acceptability of contacting materials is dependant upon
the slurry properties and the required time between
routine maintenance. Short-term batch processing can
be satisfied by standard valves which are completely
replaced on a regular basis. Long-term continuous processing
of abrasive slurries can only be handled successfully
by valves of the correct design manufactured from suitable
materials.
The abrasive
qualities of a slurry can only be established accurately
by testing. Some proprietary rotating tests have been
devised but most of the test data is unpublished. The
Miller test, ASTM G75, is a reciprocating test which
has much published data. Unfortunately, the Miller test
utilises a very hard corrosion resistant cast iron which
is not used frequently in the valve industry. A curve
is presented which allows approximate Miller Number
conversion for contacting materials of lower hardness.
Slurry
velocity is an important factor when considering wear
and material loss. At constant Miller Number wear is
directly proportional to velocity. In process pump applications
wear rates are extremely variable and generally increase
in proportion to velocity raised to the power
of 2.5 as a minimum and can be up to velocity raised
to the power of 5. Pipe wear has been correlated as
proportional to velocity raised to the power
of 3.
The abrasion/erosion
resistance of contacting materials can only be established
accurately by testing. Many researchers have adopted
different techniques to measure material properties.
The Taber test, DIN 53516, is used extensively for abrasion
testing of elastomers and plastics, but is generally
considered to be unreliable because of the clogging
of the wear drum. Testing with slurry can be complicated
and costly. Non-flowing tests can provide inaccurate
results due to attrition of the sample. Erosive testing,
using slurry jets, has provided some good test data.
Slurry jet test rigs can be "once-through"
or recirculation; attrition may or may not be a contributory
factor. Erosive jet testing has shown that material
hardness is a significant factor in combating abrasion/erosion.
Material hardness should be greater than 75% of the
particle hardness if material loss is to be restricted.
