Pump Solutions Group Neptune Series 500 Specs Industrial - Manual (Page 7)

Neptune Chemical Pump Co.
P.O. Box 247
Lansdale, PA 19446 Phone: 215-699-8700
Fax: 215-699-0370
800 Fax: 800-255-4017
www.neptune1.com
NEPTUNE/CHEMICAL FEED SYSTEM SPEC 10-18-00 jvb ZL107972
7
AUTOMATIC PROPORTIONAL CONTROL
Variable Speed vs. Variable Stroke Length
Chemical metering pumps can be controlled by flow, pH or other process instruments
using either variable speed drives or variable stroke length mechanisms.
Please understand how these two methods differ and how they may impact the
process.
Turndown – generally speaking variable stroke length adjustment will
allow the greater automatic turndown. The full 10:1 turndown available
from the metering pump is always available from an electric stroke
length adjustment mechanism. Variable speed drives are limited in
turndown either through torque loss and stalling at low RPM or by
requiring stroking speeds that are below the practical limit of the pump
operating range. We recommend that pumps not be operated below 20
spm, and poor performance can be expected below 15 spm, hence, a 40
spm pump would be limited to a 2:1 or 3:1 turndown if it were used with
a variable speed drive. Practical turn down limits of a higher speed, 120
spm pump are still only 6:1.
Adjustable Range – use of a variable speed drive does increase the
combined manual and automatic adjustable range of the pump. The
metering dial remains available as an independent adjustment. In
some applications this may be an advantage.
Slug Feed Phenomenon – an important consideration in many
applications is the uniform delivery of chemical. Use of variable stroke
length adjustment mechanisms causes a smaller dose of chemical to
be dispensed, however, the frequency of injection remains constant.
Variable speed drive results in the same size slug of chemical being
injected,. The frequency of injection is varied.
Applications which require injection in to a flowing line may be poorly
served by a variable speed mechanism because the slugs of chemical
become fewer more widely separated as the pump slows down unless
flow in the line slows proportionately.
Do be aware of these differences when choosing between automatic stroke length or
automatic motor speed controls.