Energy‑Saving Retrofit of 150ZJ-50 Slurry Pump for Filter Press Feed Service: VFD Constant Pressure Control and Minimum Frequency Optimization

Subtitle: Replacing fixed speed + recirculation valve with VFD constant pressure control achieves 35% energy savings, 8‑month payback – minimum frequency 30Hz prevents back blade failure

Introduction

In mineral processing, coal preparation, and chemical plants, the filter press feed service is a typical application for slurry pumps. The 150ZJ-50 slurry pump (150mm discharge) is widely used to feed high‑concentration slurries to filter presses. The traditional feed method employs a fixed speed pump + recirculation valve (bypass): the pump runs at rated speed while excess slurry returns to the feed tank, and only a portion enters the filter press. This “overpowered” operation wastes significant energy across the valve, causes high motor current, and accelerates valve wear.

VFD constant pressure control adjusts pump speed to match the varying demand of the filter press cycle, eliminating valve throttling losses. However, a VFD retrofit is not simply adding a drive – the minimum frequency must be set properly to avoid axial thrust imbalance and bearing overheating due to reduced centrifugal force from the back blades at low speeds. Based on successful retrofits in multiple concentrators, Hebei Xingou Machinery Equipment Co., Ltd. provides a complete VFD constant pressure retrofit solution for 150ZJ-50 pumps, including system configuration, PID tuning, minimum frequency optimization, measured energy savings, and payback analysis.

1. Process Characteristics of Filter Press Feed and Energy Analysis

1.1 Three‑Stage Characteristics of Filter Press Cycle

A complete filter press cycle typically consists of three stages:

Fixed speed pumps cannot adjust speed and rely on a recirculation (or inlet) valve to adapt to these changes. During the pressing and holding stages, a large portion of energy is wasted across the valve.

1.2 Energy Loss with Fixed Speed + Recirculation Valve

Actual operating data from an iron ore concentrator using a 150ZJ-50 pump for filter press feed:

During pressing and holding, valve losses account for 44%‑73% of total power – a massive energy waste.

2. VFD Constant Pressure Control Principle and System Configuration

2.1 Operating Principle

The core of VFD constant pressure control is: install a pressure transmitter at the filter press inlet. The pressure signal is fed back to the VFD, which automatically adjusts pump speed to maintain the feed pressure at the setpoint. As the filter press chambers fill and resistance increases, the pump slows down. During the holding stage, the pump runs at a minimum safe speed, maintaining pressure while drastically reducing energy consumption.

2.2 System Configuration

Main components required for a 150ZJ-50 filter press feed VFD retrofit:

For a 150ZJ-50 pump (typical 90‑110 kW motor), a 132 kW heavy‑duty VFD is recommended (e.g., Inovance MD500, ABB ACS880 series).

3. Minimum Frequency Optimization – Avoiding Back Blade Failure

3.1 Function of Back Blades and Low‑Speed Risk

The 150ZJ-50 impeller has back blades on the rear shroud to balance axial thrust. At rated speed (~1480 rpm), the centrifugal force generated by the back blades is sufficient to reduce back chamber pressure and minimize axial load on the thrust bearing. When speed is significantly reduced, centrifugal force drops with the square of speed – at 50% speed (740 rpm), force is only 25%, and axial thrust balancing deteriorates sharply.

Main risks of low‑speed operation:

• Axial thrust imbalance → thrust bearing temperature rises, life shortened

• Impeller axial movement → mechanical seal leakage

• Severe cases: bearing seizure, seal damage

3.2 Basis for Minimum Frequency Setting

Based on field measurements of bearing temperature at different speeds and back blade centrifugal force calculations, the recommended minimum frequency settings for a 150ZJ-50 filter press feed VFD retrofit are:

Important: Set the minimum frequency limit in the VFD parameters to prevent operator error. For applications requiring prolonged low‑flow holding, consider adding a small holding pump to avoid extremely low main pump speeds.

4. PID Parameter Tuning Method

Stable constant pressure control depends on proper PID settings. Recommended initial values and tuning procedure for 150ZJ-50 filter press feed:

4.1 Initial PID Values

4.2 Tuning Procedure

5. Field Retrofit Case Study

Background: An iron ore concentrator used a 150ZJ-50 pump for filter press feed with fixed speed + recirculation valve. During pressing, motor current was 185A, power 104kW, and valve loss was 44%. Thrust bearing temperature was 78°C (high).

Retrofit:

• Added 132 kW heavy‑duty VFD

• Installed pressure transmitter (1.0 MPa range, 4‑20mA)

• Set minimum frequency 30 Hz, PID target pressure 0.55 MPa

• Tuned PID parameters: P=1.2, I=3.0s, D=0

Measured data after retrofit:

Energy saving:

• Average power reduced from 105 kW to 74 kW (weighted average for pressing+holding)

• Energy saving rate: 29.5%

• Annual operation 6,000 hours, electricity $0.09\mathrm{/}��ℎ\to ������������=(105\text{‑}74)\times 6000\times 0.09\approx \ast \ast$16,740**

Bearing temperature:

• Before retrofit: thrust bearing 78°C

• After retrofit (min frequency 30Hz): thrust bearing 68°C (10°C drop)

Payback period: Total retrofit cost ~$5,000,�����������������������$16,740 → payback 3.6 months.

6. Economic Benefits Summary

7. Precautions

Conclusion

Filter press feed is a variable‑load application, making the 150ZJ-50 slurry pump an ideal candidate for VFD constant pressure control. Replacing fixed speed + recirculation valve eliminates throttling losses and achieves energy savings of 25%‑35%. The key to success is setting an appropriate minimum frequency (≥30Hz) to avoid axial thrust imbalance and bearing overheating caused by insufficient back blade centrifugal force at low speeds. Field case validation shows a payback period of only 3‑4 months, along with significantly reduced valve and bearing maintenance costs.

Hebei Xingou Machinery Equipment Co., Ltd. offers complete VFD constant pressure retrofit solutions for 150ZJ-50 filter press feed pumps, including VFD selection, PID tuning, minimum frequency optimization, and on‑site commissioning. For energy audits or retrofit design, please contact our technical team.