1. Cost of Ownership: While it is true that solenoid pumps are often less than half the purchase price of hydraulic diaphragm metering pumps, the total cost of ownership tells a completely different story. Hydraulic diaphragm metering pumps last over 10-15 years in the field with almost no maintenance. Solenoid pumps usually need service and replacement diaphragms every 6-9 months depending upon usage. Typically, a solenoid pump is considered good if it lasts 2-3 years in the field.
2. Balanced Diaphragm Vs Imbalanced Diaphragm: Hydraulic diaphragm pumps have process liquid on one side and hydraulic liquid on the other side to achieve balanced pressure on the diaphragm. This results in the diaphragm experiencing almost no mechanical stress which ensures a long life. On a solenoid metering pump, you have all the process pressure on one side with nothing to balance against it on the other side. This puts a lot of mechanical stress on the diaphragm and eventually causes bulging and failure.
3. Accuracy Matters: Due to the very nature of its design, hydraulic diaphragm pumps are extremely accurate throughout its life. That is the reason they are the only pumps that can meet API-675 standards. Solenoid pumps cannot because the diaphragm could bulge out or balloon even before it fails, which causes the accuracy to drop significantly.
4. High Performance: Solenoid pumps are good for low flow and low pressure applications usually under 100 psi. The duty cycle also needs to be very light. On the other hand, hydraulic diaphragm pumps can achieve flows and pressures hundreds of times higher than solenoid pumps. Hydraulic diaphragm pumps are also made for heavy duty or continuous operations 24/7.
So while solenoid pumps have their place on top of a 50 gallon drum, dispensing a couple gallons of chemicals several times a day, hydraulic diaphragm pumps are true industrial pumps for process chemicals that require the pumps to be running continuously in demanding applications.