As an integral part of onshore and offshore drilling, mud pumps circulate drilling fluids to facilitate drilling oil and natural gas wells. Mud pumps stabilize pressure and support the well during the drilling process and drilling fluids provide friction reduction and a means to remove cuttings. A leak detection system for hex pumps was created for a hex mud pump with six pistons, six suction valves, and six discharge valves. The six pistons are driven by a rotating, asymmetric cam. The system monitors the suction and discharge valves using accelerometers.
The Case for an Automated Monitoring System
Valve leaks in piston pumps are often discovered at a late stage when the leaks are so severe that they induce large discharge pressure fluctuations and create washout damage (Figure 1). When a severe leak is detected, it is localized manually by listening to the fluid modules while the pump is running but it is difficult to uniquely localize the leak and distinguish between a suction valve leak and a discharge valve leak.
Human exposure to hazards is the main disadvantage of manual detection, verification, and localization. Mud pumps convert large amounts of power and often output high pressures up to 350 Bar discharge. Additional equipment in pump rooms also generates high acoustic noise pressure levels that can exceed 100 dBA and cause health and hearing damage if humans are not correctly protected.
Valve leaks often develop quickly, so manual detection gives very little time to prepare for exchanging the defective valve(s) after the leak is detected. If the leak source is uncertain, searching for the defective valve(s) can be costly and time-consuming.
Discovering the Vibration Method
During a vibration monitoring project for hex pumps, a Norwegian oil well company discovered the possibility of detecting leaks using accelerometers. Vibrations were recorded at different locations, both on the pump and on the discharge line, along with suction pressure, discharge pressure, and pump speeds for different pump conditions. A 20-kHz sampling frequency was used and 5-second snapshots were taken with intervals of a few minutes. On one occasion, the vibration signature significantly changed during a 15-minute period; the spot was a growing valve leak.
After the initial discovery, more tests were performed to further explore the leak detection possibility. Analyzing the frequency spectra indicated that the leak induces strong, broad-banded noise from 3 kHz up to the Nyquist frequency of 12.5 kHz (half the sampling frequency of 25 kHz). The overall noise level increases by a magnitude of 30 dB.
Leak Detection System
Based on that encouraging experience, the company wanted to include this condition-based maintenance system as a standard feature on all hex pumps, so it developed the system as a standalone module to add to the existing hex pump control system. Slightly simplified, it consists of the following components: accelerometers (one per valve block), a proximity sensor picking up pump speed and phase, a discharge pressure sensor, an embedded monitoring system (an NI CompactRIO system with acquisition modules for powering the accelerometers and acquiring high-frequency data), signal processing software and alarm logics implemented using NI LabVIEW software running on the CompactRIO monitoring system, and an HMI user interface developed in LabVIEW.
The data acquisition and signal processing are briefly described by the following steps:
Capture high-rate data (25-kHz sample rate) from all sensors during a short time interval covering at least one pump cycle.
Bandpass filter the acceleration signals to minimize the influence from ambient pump vibrations.
Analyze the timing signal to find pump speed and cam angle.
Construct adjusted window functions that selectively pick the filtered acceleration signal in every valve closing phase (adjusted here means narrowed and time lag corrected so that valve closing spikes are excluded).
Use these windows to calculate the RMS vibration level for each valve closing phase.
Normalize the vibration levels through division by the median vibration level.
Set a leak alarm if one or more of the normalized vibration levels exceeds a specified threshold during a certain time interval.
The default sampling frequency of the signals is 25 kHz but the system can handle higher rates if necessary. The bandpass filter is optional but experience shows that it improves contrast and detection sensitivity. Signal strength normalization by the median vibration level makes the detection nearly independent of the inherent ambient vibrations, which increase rapidly with increasing pump speed and discharge pressure. The last requirement — that the detected leaks last for a set time — eliminates erratic alarms caused by debris or large particles that can cause temporary seal malfunction.
The leaks detected can be remotely verified automatically by signal processing in several ways. First, the operator can view and interpret the vibration signals directly from graphs. Second, the operator can selectively listen to the recorded acceleration signals as audio signals to hear the leak sound. Third, the operator can check to see if the mean discharge pressure is stable or dropping. Lastly, the operator can see if the lowest pressure harmonics are growing.
The human ear/brain is an extremely sensitive instrument for picking up abnormal sounds. If the leak sound is too far up in the treble frequency range to hear, the signal can be played back with a lower sampling rate, thus transforming the leak noise into a more audible frequency band for the human ear.
A desktop application can be used on a terminal to review the LDS and read raw logs and trend files directly from the LDS. This additional feature gives the operator the chance to get a closer view of the vibrations and perform audio playback to the user. Also, the high-rate log of the discharge pressure can be viewed to reveal a cyclic variation drop. This helps provide a better understanding of what is happening with the valves.
Figure 2 shows a diagnosis screen from the hex pump control screen delivered by the leak detection system. It shows a very clear overview of the valve status and a vibration level trend of all valves.
Based on the field experience of the new leak detection system, it was concluded that the leak detection method offers many advantages over current practices, including the following:
High sensitivity for early leak detection and localization
Remote, continuous, and computer-based pump monitoring
Increased safety through less human exposure to hazardous environments
Multiple leak detection and localization (in hex pumps)
Reduced maintenance time and cost because leaky valve(s) are localized before the valve exchange jobs start
Easy to retrofit existing pumps because accelerometers can be attached by glue, magnets, or tape
The NI tools for prototyping the system provided an embedded deployment system that can reliably retrofit to existing pumps. In comparison to other leak detection methods based on analyzing discharge pressure, the vibration-based methods are more robust and reliable, especially when it comes to localizing a leak. Studies showed that an alternative method can be applied for shaft-driven piston pumps having either an integrated valve block or split blocks with a high vibration transfer. Leak localization for this kind of pump is mainly based on the phase of the pulsating vibration level. It can be used to localize one dominating leaky valve at a time.
This article was contributed by NI, Austin, TX. For more information, visit here .